In Dungeon Crawl Classics, a character’s skill set is very much determined by his starting occupation as a 0-level character. A character is trained in anything that his occupation would have him know, and makes related skill checks using 1d20. If the character’s background doesn’t support a task, he uses 1d10 instead. If the Judge is uncertain, the character uses 1d10 and gains a +2 bonus.
I find it desirable to sometimes reflect changes in skill level. For example, a character who was a scribe might tutor his herder friend so that the herder could read a little better. Or a barbarian from Cimmeria might spend time in the Thieves’ Quarter of Shadizar the Wicked and gain some small level of skill thereby. Conversely, a beadle who hasn’t set foot in a church for many years might be somewhat rusty.
The Dice Chain can be used to simulate growth of skill, or atrophy of skills that are never used. Basically, if the Judge feels that an untrained skill is being used repeatedly in important circumstances, with serious consequences for failure, he can allow the character to make checks against that skill one step up the dice chain. For example, a gong-farmer who spends three adventures on a caravan might learn to handle camels using a d12 instead of a d10.
Conversely, if that same gong farmer avoids examining dung, even when it is relevant, the Judge may eventually have him roll checks to do so using a d16 instead of a d20. Atrophy of skills should reverse up the die chain with any success, as old habits are remembered.
It is important to keep a short leash on this idea, because, while simulating character growth is desirable, growing to the unwieldy mass of skills some other games list is not. No skill should ever surpass rolling on a d20, or fall below rolling on a d10, as a result of using this method. This should be used to add flavour only – if it becomes a headache, or a bookkeeping exercise, simply drop it!
If a character relies on a weapon she is not trained in over the course of three adventures, the Judge may, but does not have to, allow that weapon to be included in her list of trained weapons. This rule should be used more so that the Judge can introduce new, esoteric weapons to the game milieu than to allow player characters to “get around” current class restrictions. The Judge is strongly advised to determine, before the weapon is introduced, which classes may benefit from training. The Judge need not communicate this information to the players.