Quantitative metathesis (or transfer of quantity) is a specific form of metathesis or transposition (a sound change) involving quantity or vowel length. By this process, two vowels near each other – one long, one short – switch their lengths, so that the long one becomes short, and the short one becomes long. In theory, the definition includes both long-short → short-long and short-long → long-short, but Ancient Greek, which the term was originally created to describe, displays only the former, since the process is part of long-vowel shortening.