The Spanish word 'dos' meaning 'two' comes from the Latin word 'duo' (also meaning 'two'), specifically from its accusative form 'duos'. The word underwent minimal changes as it evolved from Latin to Spanish, maintaining both its form and meaning remarkably well over time.

Several common Spanish words are built from 'dos', including: 'doble' (double), 'docena' (dozen), 'dúo' (duet), and 'duplo' (double, twofold). These words all carry the concept of 'two' or 'doubling' in their meanings.

Many English words share the same Latin root 'duo', including: 'dual', 'duet', 'double', 'dozen', and 'duplicate'. The concept of 'two' or 'twice' is preserved in all these related words. Even the prefix 'du-' in English often indicates something involving two of something.