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 | Makes them one point |
| Horizontal / vertical | Z | 1+ lines | Snaps each 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 length | E | 2+ lines | The same length as each other |
| Equal radius | Q | 2+ circles or arcs | 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 |
| Point on line | N | 1 point + 1 or more lines | The point rides the line |
| Point on curve | U | 1 point + 1 or more curves | The point rides the curve |
| 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 ~3° 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 delete them. 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.
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