Form W-4
Also called: W-4, withholding certificate
The W-4 is the form telling your US employer how much tax to withhold. It was redesigned in 2020 to drop the old 'allowances' in favour of direct dollar entries, which made it more accurate and less familiar. It's worth redoing after any marriage, birth, second job or large raise — a second job is the most common cause of severe under-withholding, since each employer withholds as if its salary were your only income.