Copulative a

The copulative a (also a copulativum, a athroistikon) is the prefix ἁ- () or α- () used to express unity in Ancient Greek, derived from Proto-Indo-European *sm̥-, cognate to English same (see also symbel).

An example is ἀδελφός ( 'brother'), from *sm̥-gʷelbhos, literally meaning 'from the same womb' (compare Delphi).

In Proto-Greek, the Proto-Indo-European phoneme *s at the beginning of a word became *h by debuccalization and syllabic *m̥ became *a, giving the combined form ha-. The initial h was sometimes lost by psilosis or Grassmann's law.

Cognate forms in other languages preserve the original Proto-Indo-European *s. For example, the Sanskrit prefix occurs in the name of the language, सं॒स्कृ॒त , literally 'put together'. Less exact cognates include English same and some, and Latin simul 'at the same time' and similis 'similar'.

Other words in Greek are related, including ᾰ̔́μᾰ ( 'at the same time'), ὁμός ( 'same'), and εἷς ( 'one'; from Proto-Indo-European *sem-s).