A (musical note)

Notation and acoustics

In scientific pitch notation A4 designates the A above middle C. Using the widely accepted concert-pitch convention, A4 is 440 Hz; each octave up doubles the frequency and each octave down halves it. Thus A3 = 220 Hz, A5 = 880 Hz, and so on. On a standard 88-key piano the lowest A is A0 and the highest A is A7. In equal temperament, A has enharmonic equivalents such as A# (A sharp) and B♭ (B flat) depending on context.

Historical development of pitch standards

Pitch standards have varied historically and geographically. Baroque and early Classical ensembles often used lower reference pitches than those common today, and regional pitch pipes and organs could differ noticeably. During the 19th and 20th centuries efforts to standardize concert pitch led to A4 = 440 Hz being widely adopted as a practical international reference, though alternative standards (for historically informed performance, for example) are still used.

Role in tuning and instruments

In orchestral and ensemble settings the oboe commonly sounds an A for the group to tune to; players match their pitch to this reference before rehearsal and performance. String instruments typically include an A string (for example, the violin, viola and cello all have an A string as a core open string), which makes it convenient for players to tune relative to that open string. On fretted instruments such as the guitar, A is a standard string pitch in conventional tuning.

Keys, scales and musical uses

A is the tonic of several commonly used keys, notably A major (three sharps) and A minor (the relative minor of C major). Composers and performers treat A both as a structural center in compositions and as a reference for melodic and harmonic writing. Some technical passages or vocal lines that require high or low As can be challenging for performers depending on the octave and their instrument or voice type.

Practical notes and distinctions

• Orchestral tuning: the oboe often gives the tuning A for the ensemble to match; see an example source: oboe.
• Piano range: the lowest A on a standard piano is A0 and higher octaves proceed up to A7; see general piano information: piano.
• Voice and registers: while A is not a universal vocal milestone, certain high or low As can be demanding for singers in particular registers; further discussion of vocal ranges is available at: voice ranges.

As a single pitch class, A intersects acoustics, notation, history and performance practice. Whether serving as a tuning reference, the basis of a key, or an open string on an instrument, A remains central to Western musical organization and everyday practice.