Psi is the twenty-third Greek letter, a Greek innovation without a Phoenician model; its value is 700. Its trident-like shape has been adopted in modern times as the emblem of psychology and psychiatry (for psyche); it has no significant ancient mystical tradition beyond its place in isopsephy. The Greek letters doubled as numerals in the Milesian (alphabetic) system, in which each letter carried a fixed value; the practice of summing a word's letters into a number, called isopsephy, is the Greek counterpart of Hebrew gematria and appears in inscriptions, the Greek Magical Papyri and early Christian texts.
Psi is one of four supplementary letters, together with Phi, Chi and Omega, added at the end of the Greek alphabet once its Phoenician-derived series had run out; like Phi and Chi before it, it has no Semitic ancestor and no inherited name of its own, its name likewise built simply from its sound plus a vowel, in the pattern of Greek's other late, home-grown letters. Its isopsephy value of 700 belongs to the Milesian alphabetic numeral system, which also needed three further, otherwise obsolete letters, digamma (also called stigma), koppa and sampi, to cover every value from 1 up to 900; none of the three survives among the 24 letters in ordinary use today.