Tool: Knife and Scissors
Why does the toolbar have two tools for cutting?
It looks like an oversight — the kind of duplication that accumulates in software when two people implement the same idea in different years and nobody has the authority to delete one. It isn’t. Scissors and Knife do genuinely different things, and the difference is one of the more useful distinctions in the whole toolbar once you can see it.
Scissors cuts at a point. You click on a path, on a specific spot, and the path is split there. One path becomes two, joined nowhere, each now selectable and movable on its own. Nothing else in the drawing is affected. It’s a surgical cut with a stated location, and the location is where you clicked.
Knife cuts along a line. You drag across the canvas, and everything your drag passes through is sliced — not at one point, but along the whole stroke, and not just the path you were aiming at, but every path that happened to be in the way. It’s a cleaver, not a scalpel.
Once you have that, the choice is obvious in every real situation. If you want to open a closed shape so you can pull one side of it away — click, Scissors. If you want to take a rectangle and end up with two triangles — drag a diagonal, Knife. If you want to trim the tail off a line — Scissors at the point where the tail starts, then delete. If you want to cut a whole cluster of overlapping shapes along one straight edge, as though you had laid a ruler over them and drawn a blade down it — Knife, once, and every one of them opens along the same line.
That last case is the one that shows why the difference matters. Doing it with Scissors would mean clicking each path twice, on each side, and getting the positions to agree with each other by eye. Doing it with Knife is one gesture, and the cuts agree by construction because they are all the same cut.
Here’s the underlying distinction, and it’s a general one worth carrying around: Scissors takes a point as its argument, and Knife takes a path. Everything that follows — the surgical/cleaver feel, the affects-one-thing/affects-everything behaviour — falls out of that. When you know what a tool takes, you know what it’s for. [1]
There’s a second, quieter thing about Knife that took me a while to appreciate. It cuts through everything in its path, and that is a feature, not a hazard, because it means the cuts line up. If you slice four stacked shapes with one drag, the four resulting edges are collinear — perfectly, not approximately. You could not achieve that by hand in any amount of time, and it is exactly what you want when you’re cutting a composition apart to reveal a layer beneath, or splitting a wordmark along a diagonal so the halves can slide.
The hazard is real too, of course, which is that Knife will happily cut things you had forgotten were under the cursor. Lock a layer you don’t want touched, or zoom in so your drag is short. [2]
Both tools leave you with paths, not fragments of some new type. That sounds obvious and it is the thing that makes them composable: cut a shape in half, then reshape one half, then smooth it, then give it a width profile. A cut isn’t a special state. It’s just a document with more paths in it than before.
And if what you actually wanted was to remove a region rather than divide one, you don’t want either of these. You want Shape Builder, alt-dragging the face away, or one of the erasers — which are their own confusion, and their own post.
[1] The same lens explains most of the toolbar. Curvature takes points. Reshape takes a region. Width takes a point on a path plus a magnitude. Shape Builder takes a drag across faces. A tool is mostly defined by the shape of the argument it accepts, and its behaviour is mostly the consequence.
[2] Knife also does not care about your feelings regarding groups, which is the kind of sentence you write after losing a composition to a careless drag.