Geometric Constraints
Use a geometric constraint to hold geometry in a relation to other geometry: level, parallel, tangent, the same size. Select the geometry first, then press the key from the table below.
No numbers are involved. Those are dimensions, and they are the next article. Every constraint here takes a selection, so the constraint group appears on the toolbar only once you have made one.
The reference
| Constraint | Key | Select | What it does |
|---|---|---|---|
| Coincident | C | 2 points, or 1 point + 1 or more lines or curves | Makes two points one point; puts a point on the line or curve it was picked with |
| Horizontal / vertical | Z | 1+ lines, or 2 points | Snaps each line — or the pair of points — to its nearer axis and holds it there |
| Parallel | P | 2 lines | Same direction |
| Perpendicular | V | 2 lines | Square to each other |
| Tangent | T | 2 curves, one line with several curves, or one curve with several lines | Meet smoothly, no corner |
| Concentric | O | 2+ circles or arcs | Share a centre |
| Equal | E | 2+ lines and/or 2+ circles or arcs | The lines the same length, the curves the same radius |
| Symmetric | S | 2 items + a line | Mirrored about that line |
| Midpoint | M | 1 point + 1 line | The point sits halfway along (lines only — arcs are not accepted) |
| Fix point | F | 1 point | Pinned where it is |
| Mesh gears | — | 2 gears | Same module, pitch circles tangent, teeth clocked to interlock |
Two more build geometry rather than relate it. Add midpoint (B) drops a point at the middle of a line and constrains it there, and Add intersection (J) drops one where two lines cross.
Which one to reach for
Reach for a relation before you reach for a number. A number is a decision you have to maintain. A relation is a decision that maintains itself.
- Two holes that must stay level: horizontal through their centres, not two identical y dimensions.
- A slot that must stay centred: symmetric about a centreline, not two dimensions you have to keep equal.
- Four sides of a square: equal length plus one dimension, not four dimensions.
Every relation you use instead of a number is one fewer thing to update when the part changes, and one fewer chance for two numbers to disagree.
What tools add for you
You will find constraints you never applied. They come from the snaps you made and the tools you drew with.
- Most snaps leave the matching constraint behind, onto an axis, a line, a midpoint or a curve. Snapping onto an existing free point does something stronger and quieter. It reuses that point, so the two ends are one point rather than two points held together, and there is no chip to find. This is covered in Snapping and Inference.
- A line drawn within 5° of an axis gets a horizontal or vertical.
- Rectangles arrive with three perpendiculars and a horizontal on the base edge. That is enough, because the fourth right angle and the opposite equal lengths follow from the loop closing. The from-centre variant swaps two perpendiculars for midpoint pins on the diagonals. The three-point one drops the horizontal, since you chose the base direction yourself. Polygons arrive with equal sides and pinned interior angles, and slots with tangency and equal radii.
They show up as chips like any other, and you can remove them the same way. If a shape refuses to deform the way you want, look for one of these first, most often that horizontal on a rectangle's base, the one that stops you rotating it.
The chips on the canvas
A constraint you cannot otherwise see is drawn as a small icon where the constraint actually happens: a tangency at the point the two curves touch, ringed with a soft dashed circle because nothing else in the sketch marks it; a coincidence at the shared point; a concentricity at the shared centre; a midpoint or a point-on-line at the point; a parallel, a perpendicular or an equal length at the midpoint of each line; a symmetry at the middle of its axis.
Chips are grouped by place rather than by primitive, so one tangency is one chip however much of it you have selected. Icons that land within a few pixels of each other merge into one stack, and past six a chip folds the rest into a +N whose tooltip names them. At rest a chip is only its icons; the box around them appears under the pointer.
Clicking a chip opens it, it does not delete it. The popover names the constraint in a line, shows the solver's verdict on it — redundant, over-specified, not applied — pulses the geometry it holds so you can see what you are about to lose, and carries an explicit Delete button. A constraint that arrived with a placed smart object shows a read-only note where the button would be. There is one exception: an amber chip still deletes on the first click, because the solver has already proved that constraint frees no degree of freedom, so that click cannot cost anything. The Delete key works too, once a constraint is selected.
Chips can be dragged out of the way — the ones that have anywhere to go. A glyph pinned to a point has one place it can mean and stays there, but a glyph that belongs to a line slides along that line and takes whichever side you pull it to. The placement is stored along the line rather than as a point on the plane, so it survives a zoom, a change of plane and the geometry moving underneath it. Dragging one never touches the sketch: the document is written once, when you let go.
See also
- How the Solver Works: degrees of freedom and the status pill
- Dimensions: constraints with a number in them
- Snapping and Inference: the ones you get without asking