Welsh saith

← Proto-Brythonic *seiθ (inherited)

← Proto-Celtic *sextam (inherited)

← Proto-Indo-European *septḿ̥ (inherited)

← Middle Welsh seith (inherited)

→ Welsh deusaith (compound)

→ Welsh seithfed (affix)

→ Welsh Saith Seren (compound)

→ Welsh seithongl (compound)

→ Welsh saith ar hugain (compound)

→ Welsh un deg saith (compound)

→ Welsh dau ddeg saith (compound)