(aorist passive stem), velars become , labials become , and dentals become . Before an (perfect middle first-singular, first-plural, participle), velars become , nasal+velar becomes , labials become , dentals become , other sonorants remain the same. The alveolar nasal assimilates in place of articulation, changing to a labial or velar nasal before labials or velars: before the labials , , , (and the cluster ): ; before the velars , , (and the cluster ): When precedes or , the first consonant assimilates to the second, gemination takes place, and the combination is pronounced , as in from underlying , or , as in from underlying . The sound of zeta develops from original in some cases, and in other cases from . In the second case, it was likely first pronounced or , and this cluster underwent metathesis early in the Ancient Greek period. Metathesis is likely in this case; clusters of a voiced stop and , like , do not occur in Ancient Greek, since they change to by assimilation (see below), while clusters with the opposite order, like , pronounced , do occur. ('to Athens') < ('set') < Proto-Indo-European (Latin : reduplicated present), from zero-grade of the root of < "seat" ('on foot') < PGr , from the root of "foot" ('revere') < PGr , from the root of ('holy') Terminology Ancient grammarians, such as Aristotle in his Poetics and Dionysius Thrax in his Art of Grammar, categorized letters () according to what speech sounds ( 'elements') they represented. They called the letters for vowels ('pronounceable', singular ); the letters for the nasals, liquids, and , and the letters for the consonant clusters ('half-sounding', singular ); and the letters for the stops ('not-sounding', singular ). Dionysius also called consonants in general ('pronounced with [a vowel]', ). All the Greek terms for letters or sounds are nominalized adjectives in the neuter gender, to agree with the neuter nouns and , since they were used to modify the nouns, as in ('pronounceable element') or ('unpronounceable letters'). Many also use the root of the deverbal noun ('voice, sound'). The words were loan-translated into Latin as . The Latin words are feminine because the Latin noun ('letter') is feminine. They were later borrowed into English as vowel, consonant, semivowel, mute. The categories of vowel letters were ('two-time, short, long'). These adjectives describe whether the vowel letters represented both long and