||z ||ĵ |- !Pronunciation before a voiceless obstruent | p ||t ||c ||ĉ ||k ||f ||s ||ŝ |} In compound lexical words, Zamenhof himself inserted an epenthetic vowel between obstruents with different voicing, as in above, never , and , never as with some later writers; mixed voicing only occurred with grammatical words, for example with compound numbers and with prepositions used as prefixes, as in and above. V is never found before any consonant in Zamenhof's writing, because that would force it to contrast with ŭ. Similarly, mixed sibilant sequences, as in the polymorphemic ('to scatter'), tend to assimilate in rapid speech, sometimes completely (). Like the generally ignored regressive devoicing in words such as , progressive devoicing tends to go unnoticed within obstruent–sonorant clusters, as in ('additional'; contrasts with 'blue') and ('boy'; the kn- contrasts with gn-, as in 'gnome'). Partial to full devoicing of the sonorant is probably the norm for most speakers. Voicing assimilation of affricates and fricatives before nasals, as in ('a detachment') and the suffix ('-ism'), is both more noticeable and easier for most speakers to avoid, so for is less tolerated than for . Loss of phonemic ĥ The sound of , , was always somewhat marginal in Esperanto, and there has been a strong move to merge it into , starting with suggestions from Zamenhof himself.R. Bartholdt and A. Christen, H. Res. 415 "A resolution providing for the study of Esperanto as an auxiliary language". Hearings before the Committee on Education, House of Representatives, 63rd Congress, 2nd Session 1914 March 17. Dictionaries generally cross-reference and , but the sequence (as in 'architecture') was replaced by () so completely by the early 20th century that few dictionaries even list as an option. The central/eastern European form for 'Chinese', , has been completely replaced with the western European form, , a unique exception to the general pattern, perhaps because the word ('cinematography') already existed. Other words, such as ('chemistry') and ('monk'), still vary but are more commonly found with (). In a few cases, such as with words of Russian origin, may instead be replaced by . This merger has had only a few complications. Zamenhof gave ('chorus') the alternative form , because both ('heart') and ('hour') were taken. The two words still almost universally seen with are ('echo') and ('a Czech'). (perfective aspect) and ('check') already exist, though for is occasionally seen. Proper