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The Latin title rex has the meaning of "king, ruler" (monarch). It is derived from Proto-Indo-European *h₃rḗǵs. Its cognates include Sanskrit rājan, Gothic reiks, and Old Irish rí, etc. Its Greek equivalent is archon (ἄρχων), "leader, ruler, chieftain".
- Rex Catholicissimus (Most Catholic King), awarded by the Pope to the Spanish monarchs since 1493
- Romanorum Rex (King of the Romans), used by the German king since the 11th century
- Rex Britanniae (King of Britain), Æthelbald of Mercia (737)
- Rex Scottorum (King of the Scots), used by the Scottish king between the 11th century and 1707
- Rex Sclavorum (King of the Slavs), various Medieval Slavic rulers