A

Ajna, the Third Eye chakra, appears in the classical Sanskrit tantric sources with its own seed syllable and symbolism centred on inner sight and intuition, with no reference to Western musical pitch. Its pairing with the note A in the modern chakra-tone system follows the same ascending mapping used throughout these entries, the sixth scale degree matched to the sixth chakra, a twentieth century Theosophical and New Age construction rather than an element of the older tradition.

A carries a separate and much more concrete significance in Western music entirely apart from any esoteric correspondence: A above middle C is the international standard reference pitch, fixed at 440 Hz by the ISO 16 standard adopted in the mid twentieth century, against which every other pitch in equal temperament tuning is calculated. Some modern esoteric writers note this practical primacy and read it as reinforcing A's pairing with a chakra concerned with perception and awareness, an interpretive flourish added after the fact rather than a reason drawn from the chakra-tone system's actual twentieth century origin.

Sources The Serpent PowerSir John Woodroffe (Arthur Avalon) with Rainbow Body: A History of the Western Chakra System from Blavatsky to BrennanKurt Leland, ISO 16:1975, Acoustics, Standard tuning frequency (Standard musical pitch)International Organization for Standardization

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.)