mark vs. fold

Every crease so far has used mark. This tutorial puts the same crease expression under two different verbs so you can see exactly what each one does.

mark: record the crease

; mark.bel
paper square
mark map .a onto .b

map .a onto .b is the same expression from the last tutorial — the crease that carries the bottom-left corner onto the bottom-right corner, straight down the middle of the sheet. Under mark, Beloch records that line onto the paper and stops there. The paper stays flat: one face, with a crease drawn across it. Toggle the card to its folded view and nothing changes — there is nothing folded yet to show. mark is for laying down reference and construction creases you build other things from, without disturbing the sheet itself.

fold: move the paper

; fold.bel
paper square
fold map .a onto .b

Same expression, map .a onto .b, but now the verb is fold. Beloch still computes the same crease — but this time it also carries the paper along it: .a lands exactly on .b, and the sheet becomes two stacked layers, front and back. The folded result has 2 faces, where the mark version had one. Toggle this card to its folded view and you'll see the paper actually folded over, not just a line drawn on a flat sheet.

What you learned

• mark and fold take the same crease expressions — map ... onto ..., through ..., and the rest all work under either verb.
• mark <crease> records the line without moving the paper: the result stays flat, one face.
• fold <crease> moves the material along that line: the result is a folded state, here with 2 faces.
• The verb, not the expression, decides whether the paper actually moves.

Next, we'll look at how those layers stack and in what order: Layers & Ordering.