Procedural lockpicking

Jump to the proposed rules: general, for 5e or for B/X.


W.F. Smith of Prismatic Wasteland posted recently about making lockpicking and hacking more interactive than a simple skill roll, starting from the follow minigame:

In Errant by Ava Islam (itself the spawn of these rules from Telecanter), each lock requires a specific combination of actions: either twist, tap or turn, none of which are used consecutively. The would-be locksmith tries to guess the correct combination one action at a time. When they first pick a wrong action, it causes the lock to [feel] stiff. If a wrong action is performed on a stiff lock, the lock is jammed.

Thematically this is great fun, and combined with rules that certain kinds of locks share a combination, there’s a nice knowledge-collection and player skill component as well. Telecanter’s version has a bit of problem with probabilities—the rules for stiff locks are too permissive, so that the overall chance of success is 2/3 and the second and third steps don’t matter at all—but Errant fixes this by allowing only one wrong action across all three steps. This results in an overall 1/3 chance of success in the basic case. I think it helps to see this procedure visually:

Lockpicking procedure Flow chart of outcomes in the Errant version of the minigame.

Errant includes modifications to the game for locks of varying quality, but W.F.’s addition is reincorporate character skill:

[To] integrate it more fully into a rule system with stat-based d20 tests[, …] have the wannabe burglar make a moderate Dexterity test at the onset, with the following results:

  • On a full success, you learn the first correct action.
  • On a partial success, you eliminate one option for the first action as incorrect.
  • On a failure, you have no clue as to the first action.
  • On a critical success, the lock opens immediately.
  • On a critical failure, the lock jams immediately.

Modifying the game to reflect character skill is a great idea, and the chance of success does vary: The “failure” case is identical to the original game (a 1/3 chance of success), while the chance goes up to 1/2 on a partial success and 3/4 on a full success, although we also have to take the likelihood of success on the Dexterity ability check into account:

Example (3rd level)

Lydia Rainier is 3rd level, with thieves’ tools and a +3 Dex mod, for a total of +5 vs a moderate Dex check of DC 15. Per the optional rules in the DMG, we’ll treat 13–14 as a partial success. With a straight check, Lydia has a 65% chance of success (critical, full, or partial) and opening the lock. With the addition of the minigame, their chance of success goes down to 57.5%.

This looks pretty good! Unfortunately the likelihood of success with and without the minigame don’t always align. As character skill increases, the likelihood of success increases much more slowly when using the minigame, because the chance of coming back from a failed check of the time becomes much less common than losing the minigame after a successful check:

Example (10th level)

Years later in-game, Lydia gets +9 on the same check (from a higher Dexterity and proficiency bonus). With a straight check, Lydia has an 85% chance of success (critical, full, or partial) and opening the lock (+5% for each additional modifier point). With the addition of the minigame, their chance of success goes down to 65.83̄%.

You could argue that this difference will be offset by player skill, if Lydia’s player has been keeping track of similar locks and their combinations, but if we’re going to complicate the procedure to incorporate character skill, then +4 difference in modifiers should net more than a paltry 8.3̄% (1/12) improvement in success rate.

Fortunately, we can preserve the probabilities of the ability check by creating variations of the minigame, each with a different chance of success. I ran some simulations and eventually came up with the following, keeping the basic structure of the game unchanged.

Basic procedural lockpicking

A lock takes three steps to open. At each step, the correct action is tap, twist, or turn, and the same action is never used consecutively. Wrong actions (errors) cause the lock to become stiff and eventually jam. You can’t start over, but you can give up without jamming the lock and try again later if you gain useful information.

Versions by chance of success

Vers. Odds ~% Special rule
A 1 in 12 8 No errors allowed.
B 1 in 8 13 No errors allowed, one option ruled out in first step.
C 1 in 6 17 One error allowed but only in the first step.
D 1 in 4 25 No errors allowed, but DM tells you first step.
E 1 in 3 33 One error allowed.
F 1 in 2 50 One error allowed, one option ruled out in first step.
G 2 in 3 67 Two errors allowed.
H 3 in 4 75 One error allowed, but DM tells you first step.
I 5 in 6 83 Two errors allowed, but DM tells you 2nd step after guessing the first.
J 7 in 8 88 Two errors allowed, one option ruled out in first step.
K 11 in 12 92 Three errors allowed.


Unless otherwise specified, count errors globally, so that “one error allowed” means that over the course of guessing all three actions, an incorrect action can be selected just once (making the lock stiff) before any additional incorrect actions make the lock jam. “No errors allowed” means that the lock immediately jams if an incorrect action is attempted.

Note that the original game from Telecanter (with the rule that allows one error per step) has the same probability as if we allow two global errors (Version G). I think allowing two global error is more interesting, though, since all three steps can be dramatic—if the burglar uses up both errors on the first step, they can still choose to continue, but the pressure is on and their chance of success has dwindled. Errant’s default is Version E, and the rules from Prismatic Wasteland include E, F, and H.


These rules variants give a pretty good spread across the probability space. (All we’re missing are 5/12 and 7/12—let me know if you think of a way to include those that can be explained in one sentence.) Character skill and lock complexity can now be represented by the number of errors the burglar can recover from before the lock jams, and the options they’re able to eliminate without using up a guess. If you wanted to, you could stop here and have a nice little self-contained system: The group selects an appropriate difficulty (based on the character’s skills and circumstances and the quality of the lock), the DM secretly writes the correct combination, and the player can attempt the minigame. (Maybe a candidate for MOSAIC Strict?) But in the spirit of the Prismatic Wasteland post, let’s graft this minigame into D&D—first 5e, then B/X.

Procedural lockpicking for 5e

There are many factors that affect an ability check in 5e: the character’s ability modifier, their proficiency bonus, and the DC. Fortunately, bounded accuracy ensures that bonuses will almost always be between 0 and 11, and round DCs have common interpretations. This gives us a compact cheat sheet for success rates on ability checks, each of which maps (more or less) to one of the minigame versions above:

Ability modifier to % success and minigame version by DC

Roll mod Easy (DC 10) Medium (DC 15) Hard (DC 20) V. Hard (DC 25)
+0 F 55 D 30 A 5 – 0
+1 F 60 E 35 A 10 – 0
+2 F 65 E 40 B 15 – 0
+3 G 70 E 45 C 20 – 0
+4 H 75 F 50 D 25 – 0
+5 H 80 F 55 D 30 A 5
+6 I 85 F 60 E 35 A 10
+7 J 90 F 65 E 40 B 15
+8 K 95 G 70 F 45 C 20
+9 – 100 H 75 F 50 D 25
+10 – 100 H 80 F 55 D 30
+11 – 100 I 85 F 60 E 35

Shift up or down a letter for advantage/disadvantage.


Welby Goodbarrel, a nimble (+4 Dex) 4th level Rogue with thieves’ tools (grants proficiency bonus, +2), want to pick a well-made (DC 20) lock. Instead of calling for an ability check, the DM looks up the odds of success for a d20+6 roll vs DC 20. It’s 35% and maps to lockpicking minigame version E. The DM secretly selects a combination and says, “Guess the combination to open the lock. One mistake will make the lock stiff, and a second will jam it.”

This might seem like a lot of table lookups, but in practice you probably only have a couple of characters who’ll attempt to open locks, so for the character in this example, I could jot down, “Easy→I, Medium→F, Hard→E, V.Hard→A” just like one might do for passive perception.

Procedural lockpicking for B/X

Thief skills are already percentage-based, and there are no modifiers for different qualities of locks, so it’s straightforward to map to minigame versions. Note that the easiest version (K, three errors allowed) only has a 92% chance of success, so you may want to revert to a % chance at level 12–14.

Thief level to Open Locks % and minigame version

Level OL Vers. Special rule (repeated)
1 15 B No errors allowed, one option ruled out in first step.
2 20 C One error allowed but only in the first step.
3 25 D No errors allowed, but DM tells you first step.
4 30 D No errors allowed, but DM tells you first step.
5 35 E One error allowed.
6 45 E One error allowed.
7 55 F One error allowed, one option ruled out in first step.
8 65 G Two errors allowed.
9 75 H One error allowed, but DM tells you first step.
10 85 J Two errors allowed, one option ruled out in first step.
11 95 K Three errors allowed.
12 96 K Three errors allowed.
13 97 K Three errors allowed.
14 99 K Three errors allowed.

The usual caveats for how to think about thief skills apply here; see Robert Fisher’s “On thief skills in classic D&D” and “The B/X Thief is Good” from Gorgo Mormo, and only call for a roll (or minigame) when the lock is challenging and failure would be interesting.


Date Comment
2021-05-19 Substantial revisions to the discussion about precursor rules.
2021-05-18 Posted initial draft.

Mausritter character sheet for online play


Like many of us, I’m playing more games while social distancing, and all of those games are online—mostly in Discord or Google Meet. I’ve been working through a backlog of great systems (like Trokia! and Moonhop) and next up is Mausritter.

The game’s inventory system features an arts-and-crafts, paper Tetris approach to management, and I think it’s important for players to be able to experience that, so in lieu of asking folks to print things at home, I’ve made a version of the character sheet and item cards in Google Sheets which you can copy here or by clicking the image above. (You can also just view it here, if you want to leave comments.) The original character sheet is nicely designed, so I’ve tried to stay as true to it as possible.

Update: Mausritter’s writer Isaac Williams has a post about running Mausritter online, which I’d not seen and offers some other options. There’s also a larger set of item and condition card images you can download there and use with this sheet.

Alphabetical adventures (A–B)


I’m starting up a new 5th edition campaign for a coworker and friends, as part of a New Year’s resolution to play more games in person. I went into session zero mostly blind, so to prep, I decided to start reading through the 5e Monster Manual to come up with a potential scenario for each monster with CR 3 or less. I only got through letter B before I had to move on to other prep, but it was a fun exercise which I think I’ll continue.


A shard of the Rod in Seven Pieces lies on the roof of the castle’s highest tower. A raging storm prevents anyone from getting close, but a flock of Aarakocra will do anything to retrieve it.

A wizard is trapped and wounded by a spell of her own design, within her secluded tower. With her resources limited, she communicates with the outside world through Animated objects, crudely enchanted to “bring help” or “bring a mortal soul” to free her.

A farmer lures Ankhegs to attack his rivals’ farms, but now they’ve learned to demand tribute (and there are a lot of them).

An artificer has bound an Azer to the Material Plane and makes it work the furnace. The shop produces the finest weapons you’ve ever seen, but the surrounding land for 30 miles around has become a desert.


The Death Palace of Endon is built from the petrified bodies of thousands of his subjects and foes. His beloved Basilisks, which he bred for sport, still live within, guarding his treasure.

A legendary illness has returned to the land, and the only cure grows in a glade guarded by a Spectator Beholder. Ironically, the creature was summoned a generation ago by a great medical scholar, to ensure that the cure would be available in just this situation…

Some misguided druids perform regular sacrifices over the stump of a great tree. Now there is new growth, and Blights carrying echoes of the sacrifices prowl the night.

It’s a reverse siege: Bugbear mercenaries burned a path to the middle of town, saw their warlord fall to a lucky arrow, and promptly quit the fight. Now they control the town square and the inn, and they’re almost out of beer.

Baron Uurk Urgit, a Bullywug, thought he’d stolen a shiny bauble for his queen, but that’s a fire elemental in a delicate jar. Get it back before someone breaks it!

Flip to continue: B-side sessions


My regular gaming group prefers to play our main campaign only when every player is available. Scheduling is hard, so we’ve canceled a lot of sessions over the years as a result. The past few months though, we’ve kept to our schedule more often because we’ve introduced a backup game. This game is:

  • Run by me, so that the main campaign’s GM can PC (and also because I like to GM)
  • Set in the same world and time as the main campaign
  • Run as a series of one-shots, with weaker continuity than the main campaign, and
  • Thematically distinct from the main campaign.

We call it the B-side game, and hitting all of the bullets above each time we play is a fun little side project for me. While my group started this to deal with scheduling challenges, I’d recommend this format for any group that wants a change of pace, especially if there’s a player who’s looking to experiment or build up skills at running one-shots with little prep. Here’s how we do it.

Tag in a backup GM

Or even better, a couple of them, since you want to be able to play this game when a subset of your players are availble; if you can avoid having any single player be a lynchpin, you’ll get to play more often.

It’s especially valuable to play your B-side game when the main campaign’s GM is available to play, so that they have a chance to experience their own game world. I also recommend sticking to the same ruleset for the same reason, and so that there’s less of a mental shift required if you switch from A-side (main campaign) plans to B-side last minute.


Setting the B-side game in the same world as the A-side lets your group enrich the setting and explore paths that the main party didn’t follow. As the GM, you also get the benefit of free plot hooks, recurring NPCs, and lore, courtesy of the main campaign. You also leave open the possibility of crossovers.

In our game, each player has a B-side character that they reuse between B-side sessions, although they’re welcome to change at any time.

One and done

B-side sessions should be self-contained, so that they never interfere with scheduling the main campaign. This requires forethought to pick an appropriate starting point and discipline in guiding the session to a satisfying conclusion.

There’s often an impulse to over-script a one-shot, but try to fight this. My trick is to make the first scene so open-ended that I can’t predict what the players will do. We collaborate from there, and in the last hour of our planned game time, I’ll start actively looking for ways to bring the session to a close.

All the usual one-shot tricks apply: Start in medias res, cut scenes agressively, pit the characters against the clock, and don’t pull your punches.

If your players reuse their characters between B-side sessions and you use the tricks above, weak continuity is a natural consequence. Even if a session ends with a cliffhanger, seriously consider jumping in time or space for the next session. Blank spaces and apparent contradictions are interesting.


Besides ensuring that we play more often, my goal for the B-side game is to complement the A-side game in a way that’s exciting for the players and doesn’t compromise the main campaign’s status as the main game. While organizing play into one-shots help with this, the tool I employ most is using setting, characters, and plot to push the tone of the game away from the tone of the A-side.

We’re playing Starfinder. Our A-side crew is a party of semi-professional space pirates, chasing down an ancient super weapon while getting in the middle of some complicated interplanetary politics: Great stuff for a long-running campaign, with a lot of space opera and space horror influences on a grand scale.

For the B-side game, we stick to smaller stakes. So far, the party has been down on its luck, stuck on a desert planet. While there are still big action moments, I’m drawing on cyberpunk and post-apocalyptic tropes, and aiming for a tone like the slow episodes of Cowboy Bebop. It’s punchier, more comic, and generally less real than the A-side campaign, which makes it natural for the players to want to flip back to the A-side by default.

Roll for fluency

My friend Crypt Thing from Swords Against Dungeons gave me a shoutout in an article about fantasy languages last month, so I figured I’d chime in on the subject. CT’s post touches on a lot of topics, but I’ll just pull out a few of them.

Keeping Common

First and foremost, “Common” is replaced by regional languages.

Common gets a bad rap for at least two related reasons, both of which are reasonable reactions to the way Common is used in many classic fantasy settings.

First, there’s a family of arguments that say that glossing over linguistic differences harms the setting and makes the game world less “rich” or believable, because language should be one of the most obvious markers of difference and interest in games where travel and travelers otherwise take center stage. I’ll call this the “Common is unrealistic” argument.

The second family of arguments takes this further and says glossing over language in this way is actively harmful because of how easily Common can make one culture or set of viewpoints the default. Under this view, it’s not just that Common is unrealistic, or even boring, but that Common has a homogenizing effect that erases differences.

To the “Common is unrealistic” camp, I’d say that there’s more precedent to this than you might think: Throughout history, disparate groups have settled on common languages to use for trade or government purposes while still retaining their native languages. This is often called a lingua franca. (The TVTropes page on the Common Tongue actually covers this fairly well.) If you imagine your setting’s Common as a trade language, or the language imposed by an empire that fell a couple hundred years before, it’s totally reasonable that many people would speak it, at least in the circles that adventurers tend to move in.

To the “Common is homogenizing” camp, we need to retcon Common so that it becomes a reminder to celebrate differences instead of ignore them. This post offers a nice fix along these lines to avoid falling into the trap of human = common = default, suggesting that you make Common a trade language that developed from a pidgin of multiple major languages in your setting. If you must have languages that are named after fantasy species, then just adding “Human” alongside “Elven” and “Dwarven” (as separate from Common) can be a step in the right direction.

The magical language faculty

[S]tart thinking of other factors in your world: magic, monsters, and the influences they will have in language as a whole

Mateo Diaz Torres’s A Most Thoroughly Pernicious Pamphlet has my favorite take on fantasy languages in a gaming book. It’s also fun to see how the inventory of languages evolved over time.

In particular, I like the idea of painting the world with language the way that we paint it with magic. What if mortal children could speak the language of the Fey? What if it were impossible to learn Thieves’ Cant until you’ve committed an irrevocable crime?

Language in play

Does this mean you could get a party where nobody speaks the same languages? Absolutely. […] I think that, unless the player wants that challenge, that it’s something to avoid.

I’m in favor of rolling in the open, sharing information with players that their characters don’t know, and generally doing whatever I can to keep information flowing around the table. To that end, I agree that characters ought to be able to communicate with one another, and tend to make it a requirement that everyone speaks mutually intelligible languages, on par with the requirement that the characters start with a reason to work together, rather than just having them meet up in a bar. (“You look trustworthy!”)

That said, navigating the world and conversing with NPCs is another matter, and I’d have no problem with the party deciding that they don’t really speak the language where they’re adventuring.

One alternative to making that kind of decision up front is to let the dice decide. This is how language skills work in Lamentations of the Flame Princess; language knowledge is determined at the time the language is encountered in-game. For some related thoughts, Necropraxis has a good article.

There’s also some article out there which I haven’t been able to track down, but I recall it makes a point about identifying languages, and how any adventurer worth their boots would have at least passing familiarity with the languages that are prevalent in their world.