The word "seis" uses 4 letters: eiss.
seis is a valid word in this word list. For a definition, see the external dictionary links below.
Words formed by adding one letter before or after seis (in bold), or to eiss in any order:
b - bises l - isles i - issei u - issue m - mises seism semis r - rises sires f - seifs e - seise h - shies c - sices d - sides k - sikes skies n - sines p - sipes spies s - sises t - sites sties x - sixes z - sizes v - vises w - wises