The Spanish word 'seis' meaning 'six' comes directly from the Latin word 'sex', which also meant 'six'. The evolution from Latin 'sex' to Spanish 'seis' shows a typical sound change pattern in Spanish where an 'e' was inserted between 's' and 'x', and the final 'x' sound evolved into 's'.

Some common Spanish words derived from 'seis' include 'sesenta' (sixty), 'seiscientos' (six hundred), and 'dieciséis' (sixteen). These numerical terms all incorporate the root 'seis' as their base.

The English words 'six', 'sextet' (a group of six), and 'sextuple' (sixfold) are all related to Spanish 'seis' through their common Latin ancestor 'sex'. You can see how English preserved the 'x' sound from Latin, while Spanish transformed it to 's'. The prefix 'sex-' appearing in words like 'sextant' and 'sexagonal' also comes from this same Latin root.