Contour is an instrument you play by walking. Four neural decoders sit at the corners of a square. Your finger somewhere in the middle is a blend of all four, and the square is drawn as a survey sheet. The contour lines are not decoration. They come from what those four decoders actually produce, laid out so that equal distances on screen are equal perceptual distances in sound. Cross a line and the sound has changed by a measured amount. THE SHEETS Eleven of them, each a different set of four corners, each chosen by measuring the whole library rather than by ear. They are grouped by what the measurement found: BRIGHT — HEHUAN, the brightest, where all four corners hold their own. CONTRAST — BEIYIDUAN, the widest corner spread at 8.2 times. BREATH — NENGGAO, the breathiest. DEPTH — MABO, the deepest at 53 dB. RANGE — YUSHAN, the widest at 66 semitones; JIAMING, the most focused at 17. They are named after Taiwan's high mountain traverses. PLAYING IT Touch the sheet to sound it. The ribbon underneath sets pitch, or send MIDI and play it from a keyboard. Press harder and the note opens up — the contact patch of your finger is the dynamic. Hold on empty ground to drop a mark you can come back to. The readout above the ribbon tells you where you are while you are there: brightness, noise, the grid reference, the frequency. IN A HOST Contour is also an AUv3 plug-in. It runs in AUM, GarageBand, Cubasis, Logic for iPad and anything else that hosts Audio Units, with the position automatable as two parameters and the sheet recallable with your session. The plug-in draws the same map the app does, laid out for the window a host gives it. WHAT IS FREE Three sheets — BEIYIDUAN, NENGGAO and YUSHAN — with the whole instrument: the map, the ribbon, MIDI, reverb, marks, and the plug-in. No time limit and no account. CONTOUR PRO A single purchase, once, unlocks: • All eleven sheets • Recording, straight to a file • The envelope panel — attack, decay, sustain, release • Composing sheets of your own from the library, 4,845 combinations of four corners, with the map baked for the pitches they share Requires iOS 16 or later. iPhone and iPad.