Imposition is the quiet math between "customer wants 500 cards" and "press runs 21 sheets." Get it right and jobs are profitable and boring. Get it wrong and you're either wasting paper or trimming into artwork.
The fit calculation
Take the sheet, subtract the margins your press and bindery need, then see how many pieces fit with a gutter between them: columns = (printable width + gutter) ÷ (piece width + gutter), rounded down; same for rows. Try the piece upright and rotated 90° and keep whichever fits more. A 3.5×2" card on a 12×18 with 0.25" margins and a 0.2" gutter: 24-up upright beats 20-up rotated. That one rotation check is free money.
Gutters and margins are conventions, not constants
The gutter exists so crop marks have somewhere to live and the cutter has somewhere to land; the margin exists for grippers and guides. Every shop settles on its numbers — 0.2" gutter and 0.25" margin is a common digital standard — and consistency matters more than the exact value, because your bindery's muscle memory is calibrated to it.
Crop marks: short, offset, outside
Marks sit in the gutter at each piece's corners, offset from the trim (0.125" is a widespread spec) so the blade doesn't leave half a mark on the finished piece. When bleed runs into the gutter, the marks stay clear of it — a mark printed over bleed is a mark you can't see.
Utilization is the money number
Finished-piece area × pieces-up ÷ sheet area = the fraction of the paper you're selling versus throwing away. That 24-up card layout uses 78% of a 12×18. If a different sheet gets you from 71% to 85%, that's a real margin difference on every run — which is why good imposition software reports it on every plate.