A as the Tuning Standard

A440, the A above middle C tuned to exactly 440 Hz, is the internationally standardised reference pitch under ISO 16, adopted formally in the mid twentieth century after considerable historical variation in what frequency orchestras and instrument makers treated as standard pitch; earlier historical tunings across different periods and regions used reference pitches that could sit meaningfully lower or higher than 440 Hz, which is one reason recordings and instruments built to historical tuning standards can sound subtly different in overall pitch from a modern equal-tempered ensemble playing the identical written notes.

Every other pitch in twelve-tone equal temperament is calculated as a fixed mathematical ratio from this single reference point, doubling in frequency with each ascending octave and dividing the octave into twelve equal semitone steps, which is why fixing A's frequency by international standard is sufficient to fix the frequency of every other note in the system without needing a separate standard for each.

Sources ISO 16:1975, Acoustics, Standard tuning frequency (Standard musical pitch)International Organization for Standardization with The New Grove Dictionary of Music and Musicians, 2nd editionStanley Sadie and John Tyrrell (eds.)