A

A is the universal tuning reference. The A above middle C vibrates at 440 Hz — the standard concert pitch used worldwide to tune orchestras, pianos, and fretted instruments. A major sounds bright and assertive on guitar, with the root and fifth ringing on open strings; A minor, with no sharps or flats, is the gentlest of the minor keys — reflective rather than dark.

The 440 Hz standard was adopted internationally in 1955 by the ISO. Before that, concert pitch varied widely — Baroque ensembles often tuned to A=415 Hz, while some 19th-century orchestras pushed as high as A=460 Hz.