in sufficiently obscure words, such as in "Thlaspi" (a genus of herb), and Aztec deities such as ('Tlaloc'). (The phonemes are presumably devoiced in these words.) As this might suggest, greater phonotactic diversity and complexity is tolerated in learnèd than in quotidian words, almost as if "difficult" phonotactics were an iconic indication of "difficult" vocabulary. Diconsonantal codas, for example, generally only occur in technical terms, proper names, and in geographical and ethnic terms: ('a conjunction'), ('Arctic'), ('isthmus'). However, there is a strong tendency for more basic terms to avoid coda clusters, although ('hundred'), ('after'), ('holy'), and the prefix ('ex-') (which can be used as an interjection: 'Down with the king!') are exceptions. Even when coda clusters occur in the source languages, they are often eliminated in Esperanto. For instance, many European languages have words relating to "body" with a root of . This root gave rise to two words in Esperanto, neither of which keep the full cluster: ('a military corps') (retaining the original Latin u), and ('a biological body') (losing the s). Many ordinary roots end in two or three consonants, such as ('a bicycle'), ('a shoulder'), ('a needle'), ('to cut'). However, these roots do not normally entail coda clusters except when followed by another consonant in compounds, or with poetic elision of the final -o. Even then, only sequences with decreasing sonority are possible, so although poetic occurs, *, *, and * do not. (Note that the humorous jargon does not follow this restriction, because it elides the grammatical suffix of all nouns no matter how awkward the result.) Within compounds, an epenthetic vowel is added to break up what would otherwise be unacceptable clusters of consonants. This vowel is most commonly the nominal affix -o, regardless of number or case, as in ('a songbird') (the root , 'to sing', is inherently a verb), but other part-of-speech endings may be used when -o- is judged to be grammatically inappropriate, as in ('expensive'). There is a great deal of personal variation as to when an epenthetic vowel is used. Allophonic variation With only five oral and no nasal or long vowels, Esperanto allows a fair amount of allophonic variation, though the distinction between and , and arguably and , is phonemic. The may be a labiodental fricative or a labiodental approximant , again in free variation; or , especially in the sequences kv and gv ( and , like