Customs as Arrows in a Player's Quiver

 Many players, famously, are uninterested in the minutiae of worldbuilding and social customs. In dungeon games, they want to tell a story together, and while drawing on a shared setting can elevate that, reading background is not collaborative. To that end, I propose refocusing those things DMs want players to be able to incorporate in games by explicitly describing how a PC can use a social custom. This also serves as fair warning to the players about what socially-adept NPCs may attempt. The format:

Name: Generic Name (any additional names in parentheses)

• Purpose: half-sentence starting with "to"
• Custom: second-person instruction on what is done and the consequences for breaking the custom.

Arthur Rackham

And examples:

Name: Parley (pax, formal ransom)

• Purpose: to make conflict less deadly
• Custom: if you throw your weapon aside and invoke this custom, you must be taken prisoner and have your needs looked after. Nobles are to be given means equal to their station. Killing someone who has invoked parley makes you an outlaw in the eyes of authority, and honorable underlings may help mistreated prisoners escape.

Name: Formal Gathering (promenade, seasonal feast)

• Purpose: to show respect for nobles
• Custom: if you throw a party worth 10 gp or more, anyone you invite up to minor nobility must attend and talk to you unless they have a defensible reason. When you throw a party worth 50 gp or more, anyone you invite up to a king must attend. Those who break the custom are poo-pooed by their peers and random encounters on their estate have fewer guards for a week.

Name: Debate (inquisition)

• Purpose: to inquire about a figure's belief
• Custom: while a guest drinks your beer, wine, or tea, you are entitled to ask them questions about their divine and temporal allegiances, as well as questions of religious and political doctrine. Unless they baldly lie, you can always tell their vague attitude and feeling. If they baldly lie, they are afflicted with a minor curse until the next sunrise.

Arthur Rackham

Name: Challenge of Arrows (duel, Trial)

• Purpose: to bring someone to justice
• Custom: If you publicly impugn someone thought to be virtuous, they have to come and answer this slander with contest of Bowry. The more evidence you have, the closer you get to start to the target. If they lose, or if they don't show, the law acknowledge their wrongdoing and punishes them accordingly. If they win, you risk your reputation.

Name: Sanctuary (formal presentation)

• Purpose: to respect the sovereignty of rulers
• Custom: If you present yourself to the leader of a city, town, or temple and promise good conduct, they must take care of you as long as you are not below their notice. If they bring you to harm or hand you over to someone pursuing you, they lose the right to sanctuary themselves.