The Ancient Greek diphthongs <αι> and <οι> were transliterated into Latin as <ae> and <oe>. The ligatures æ and œ were introduced when the sounds became monophthongs, and later applied to words not of Greek origin, in both Latin (for example, "cœli") and French (for example, "œuvre"). In English, which has adopted words from all three languages, it is now usual to replace "Æ/æ" with "Ae/ae" and "Œ/œ" with "Oe/oe". In many words, the digraph has been reduced to a lone "e" in all varieties of English: for example, "oeconomics", "praemium", and "aenigma". In others, it is kept in all varieties: for example, "phoenix", and usually "subpoena", but Phenix in Virginia. This is especially true of names: "Caesar", "Oedipus", "Phoebe", etc. There is no reduction of Latin -ae plurals (e.g., "larvae"); nor where the digraph <ae>/<oe> does not result from the Greek-style ligature: for example, "maelstrom", "toe". The British form "aeroplane" is an instance (compare other "aero-" words such as "aerosol"). The now chiefly North American "airplane" is not a respelling but a recoining, modelled after "airship" and "aircraft". The word "airplane" dates from 1907, at which time the prefix "aero-" was trisyllabic, often written "aëro-".