Rails

The networks that carry the money, and the rules that come with each. Cards ask permission before taking money and can be disputed for months; ACH asks nobody and tells you it failed days later. Most rail mistakes come from applying one rail's assumptions to the other.

📄️Cards

Cards are the rail the rest of this primer assumes. When a page here says "the payment," it means a card payment unless it says otherwise, so this page states the properties the other pages take for granted.

📄️ACH

ACH is the batch-based bank transfer rail in the US. It is cheap, slow, reversible, and nothing like a card payment, which matters because both often sit behind the same checkout button.