A coinoath or the act of coining is the commonplace spell used in the absence of minuted currency in most Citadel trade. While trading in goods and services isn't entirely unknown a practice, it's sometimes difficult between two parties that don't trust each other. Instead, in commerce, you make use of a coinoath.
The spell is performed by two people locking hands (or, in the absence of hands, any other available limb with skin) and the party who is selling their wares painting the oath-tier on the other person's wrist. There are local variations, but you usually utter something along the lines of "The deal is struck", while mana is being guided into the tracing, tattooing it onto the skin.
The principle is simple. The buyer promises to give something of equal value to the one who has sold them the item, and depending on the tier they have so and so many days to do it. A tier 1 oath is usually considered a rather flimsy promise, with endless timespan, while a tier 100 is a matter of life and death. If you break the oath, the tattoo will be seared into your flesh, and turn into a scar impossible to remove by natural or magical means; thus branding you as an unreliable trader.
In the matter of a tier 100 oath, breaking it might very well mean you'll lose your life.
Different traders use different tiers, and will often sign with some sort of insignia in order to mark who the debt is owed to. On rare occasion, the punishment can be altered into something of the seller's preference.
It's said that coining came into practice with old magi-slavery, practiced in the Ivory Tower.