What it is
Côtes de Genève (in English: Geneva stripes or Geneva waves) is a decorative striping pattern applied to the top surface of movement bridges, the visible side of rotors, and sometimes plates. It's made by passing a rotating abrasive wheel in straight parallel lines across the metal, leaving a series of overlapping circular cuts that read as parallel waves at viewing distance. The pattern doesn't affect function; it's decoration.
How it differs from other patterns
Côtes de Genève: parallel straight stripes (the most common). Côtes circulaires: concentric circles, used on rotors. Soleil ('sunburst'): radiating lines from a single centre, used on dials and some sub-dials. Perlage: a pattern of small overlapping circles, used on movement plates and the inside surfaces of bridges. Frosted: matte powdered finish, used on Lange and some German haute pieces.
Quality tiers
Mass-market 'Côtes de Genève' on ETA-based movements is machined: a programmed wheel passes over the bridges at fixed depth and stride. The pattern looks correct from across a room but reveals tool marks under loupe inspection. Hand-applied Côtes de Genève (used by Patek, Lange, AP haute references) requires the watchmaker to draw the wheel by hand at varied pressure, producing a more organic, less perfectly aligned pattern that's recognisably hand-finished under magnification.
Why it's everywhere
Côtes de Genève became the default Swiss movement decoration because it's relatively easy to apply at scale, looks immediately 'finished', and signals 'this is a Swiss watch'. It's now applied to movements from CHF 1,000 entry-tier (Tissot, Hamilton, Longines) up through haute-horlogerie tier (Patek, Lange), though the application quality differs enormously across that range. See wiki: Côtes de Genève.