| acquaintance | Only front vowels, last vowel rounded. | ismerős-ök |- | papír | paper | Back vowel with neutral vowel | papír-ok |} As can be seen above, the neutral vowels are able to be in both front and back vowel assonance words with no consequence. However, there are about fifty monosyllabic roots that only contain , , or that take a back vowel suffix instead of the front vowel suffix. {|class=wikitable style=text-align:center |+ Irregular suffixation ! Stem ! Gloss ! "At" ! "From" |- | híd | bridge | híd-nál | híd-tól |- | cél | aim | cél-nál | cél-tól |} These exceptions to the rule are hypothesized to have originated from the roots originally having contained a phoneme no longer present in modern Hungarian, the unrounded back vowel , or its long counterpart . It is theorized that while these vowels merged with or , less commonly or , the vowel harmony rules sensitive to the backness of the original sound remained in place. The theory finds support from etymology: related words in other languages generally have back vowels, often specifically unrounded back vowels. For example, nyíl 'arrow' (plural nyíl-ak) corresponds to Komi ньыл , Southern Mansi . Assimilation The overall characteristics of the consonant assimilation in Hungarian are the following: Assimilation types are typically regressive, that is the last element of the cluster determines the change. In most cases, it works across word boundaries if the sequence of words form an "accentual unity", that is there is no phonetic break between them (and they bear a common phase stress). Typical accentual units are: attributes and qualified nouns, e.g. hideg tél ('cold winter'); adverbs and qualified attributes, e.g. nagyon káros ~ ('very harmful'); verbs and their complements, e.g. nagyot dob ('s/he throws long toss'), vesz belőle ('take some [of it]'). There are obligatory, optional and stigmatized types of assimilation. The palatal affricates behave like stops in assimilation processes. Therefore, in this section, they will be treated as stops, including their IPA notations and . Voice assimilation In a cluster of consonants ending in an obstruent, all obstruents change their voicing according to the last one of the sequence. The affected obstruents are the following: In obstruent clusters, retrograde voicing assimilation occurs, even across word boundaries: {| class="wikitable" |- ! Voiced ! Voiceless ! Undergoes devoicing ! Undergoes voicing ! Causes voicing ! Causes devoicing |-