ACH (Automated Clearing House)
The US network for bulk bank transfers – payroll, bills and direct debits, settled in batches rather than one at a time.
It took shape in the early 1970s, when American banks realised paper cheques were about to bury them. Nacha writes the rules; the actual clearing is run by two operators, the Federal Reserve and the big banks' The Clearing House. Payments travel in files whose format has barely changed since – fixed 94-character records – and settle on a net basis a few times a day.
Almost every American paycheque arrives as direct deposit over ACH. Same Day ACH only arrived in 2016, which tells you something about the pace. The system is cheap but slow, and a debit can bounce back days later with a code like R01 (insufficient funds). That is why US fintech apps so often sit on your money for a week before releasing it.