Why are there two types of checks?
Max Berlin has been a bartender for most of his adult life, and he's currently a manager at Papillon, a French Bistro in Midtown Manhattan. He's also a friend. So that's nice. And, he's willing to answer most pressing questions for me about the industry, even if they're kind of dumb.
For the record, Berlin did confirm that there are subtle differences between the seemingly identical "Merchant" and "Customer" checks.
"The 'Merchant Copy' has an agreement just below the signature line that states the customer agrees to pay the amount shown on the receipt," he said. "The customer copy will not have this agreement, but it may have a friendly message, a 'thank you,' a tip percentage calculator... or all of the above. So the 'merchant copy' is basically designed to be a contract that is signed for the merchant records, and the 'customer copy' is used to for customer records."
For bookkeeping purposes, on both ends, is it actually important to sign the correct version?
"No. As long as there is proof that the patron has agreed to pay the amount, it's good. A simple scribble of a name below the total amount is all that is needed," he confirmed. "It's just about giving you the option, if you want it, and making it easier."
Basically, restaurant billing systems give out two distinct versions of checks to people, as a courtesy. While it doesn't logistically matter if you sign the one labeled merchant or customer, point of sale systems are designed to give out two versions just to make the choice easier, and to allow slight levels of customization (like the friendly goodbye messages, and the shockingly handy tip calculators). If you want to take one home and keep it for your records, they make the choice simple. On the merchant copy, they make sure to put the verbiage this mini-contract needs (to cover their own bases), it's up to you to decide to sign that one, specifically.
This system was a designed as an act of customer convenience. It's like labeling identical pairs of underwear for specific days of the week (what you don't do that?). It doesn't really matter, but it eliminates the terrifying possibility of choice. People are bad at making decisions, especially in public, especially after a few beers. Instead of just handing you two identical checks and letting you toil over which one to sign, the system has made "easier" for you by putting (basically, inconsequential) labels on them.
Or at least… it's supposed to be easier for you.
Bottom line: sign the customer check if you like. Sign the merchant check if you'd like. They are only doing this for you! But at the end of the day, it definitely does not matter.
Tell your parents, OK?