BY HANDCHILL

Euclid

A daily geometry proof puzzle

No typing, no memorising the format. Tap an angle in the figure, pick a theorem you recognise, and the proof writes its own next line — until the last symbol lights up: Q.E.D. ∎

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Coming soon

∠AOC = ∠BOD · vertical angles
AB ∥ CD · alternate interior angles
△ABC ≅ △ADC · SSS congruence

Three taps, one proof

  1. Tap the angles, segments, or triangles you want to use for this step.
  2. Pick a theorem below — vertical angles, alternate angles, SAS congruence…
  3. If it holds, the proof gains a line. Reach the goal and the proof is done.

A wrong turn costs you nothing — it just tells you that path is closed. This is a puzzle, not an exam.

Features

Write proofs with your thumb

The whole argument is built by tapping. One hand is enough on a crowded train.

From vertical angles to circles

38 hand-built levels across six chapters, plus a bank of 5,809 puzzles. Every one is machine-verified to be solvable.

Pick it up, put it down

No countdown, no streak to break. Close it at your stop; your progress waits for you.

Works without a signal

Solve on a plane or in a basement. Your progress syncs the moment you are back online.

Eight languages

English, 繁體中文, 简体中文, 日本語, 한국어, Español, Français, Deutsch.

Like writing in a notebook

Warm paper, hand-drawn ink, serif type. Quiet, in the best way.

“I used to dread geometry proofs because of all the formatting.
Take the formatting away and what is left is the pleasure of reasoning.”