The Spanish word 'regla' comes from the Latin word 'regula', meaning 'ruler' or 'straight edge'. 'Regula' itself was formed by combining the Latin verb 'regere' (meaning 'to rule' or 'to direct') with the suffix '-ula', which was used to form diminutives or instrumental nouns. So literally, a 'regula' was a small instrument used for ruling or directing.

Several common Spanish words share this Latin root 'regere'. For example, 'regular' (regular), 'régimen' (regime/diet), and 'dirigir' (to direct) all connect back to the basic idea of ruling or directing. The word 'rey' (king) is also related, as a king is one who rules.

Many English words are cousins to 'regla' through the same Latin ancestor. 'Regular', 'regulate', and 'ruler' all come from Latin 'regula'. The words 'reign', 'regal', and 'direct' all trace back to the Latin verb 'regere'. Even the word 'right' (as in 'correct' or 'straight') is distantly related, coming from the same ancient Indo-European root that gave us 'regere'.