Keyboard
The piano layout: white keys named by letters, black keys in alternating groups of two and three.
The black keys are the map. They repeat in groups of two, then three, over and over, and that pattern is what lets you find a note without counting from the end of the instrument.
C is the white key immediately to the left of any group of two black keys. F is immediately to the left of any group of three. Find those and the rest follow alphabetically.
Neighbouring keys, counting black and white alike, are a apart. Twelve of them make an and the pattern starts again — a full-size piano repeats it just over seven times.
The layout makes the irregularity of visible: E–F and B–C are the two white pairs with no black key between them.