A blank sheet

Straightedge and compass, and nothing else.

Every point you make is named. Press anything with the hand and the sheet tells you what it is and how big it is. Nothing here is graded except the four constructions below, and only when you press Check.

The instruments

Point

Press to set one. On a circle or a line it sticks there; where two figures cross it fastens exactly to the crossing.

Straightedge

Press one point, drag to another. Joins, never measures — Postulate 1. Hold the button down, or drag on it, and it offers a ray or a whole line instead, carried on off the sheet — Postulate 2. With the button focused, the arrow keys do the same.

Compass

Press for the spike, drag to open. Bring the opening near a point and it fastens to it exactly, saying through B before you let go — Postulate 3.

Angle

Three points: one arm, the corner, the other arm. The reading follows the figure as you drag it.

Hand

Drag anything. Drag the sheet itself to move around it. Press without dragging to ask what it is.

Getting more room

Pinch on a trackpad or roll a mouse wheel to zoom, anywhere on the sheet. Fit frames everything you have drawn; +, − and 0 do the same from the keyboard. Constructions run off the edge all the time — the crossings you need are still there when you zoom out to them.

Four constructions

Each one hands you its givens and replaces what is on the sheet. Undo brings your work back.

Notes

The sheet remembers itself in this browser, so closing the tab does not lose your work. Clear empties it; Undo goes back a step and works with ⌘Z.