The disappearance of weak yers and the change of strong yers is reconstructed by Havlík's law. The back yer (ъ) was vocalized as "e" in the Proto-Slavic basis of Western and Eastern Slovak ("pętъkъ" → "pátek:" Friday) and as "o" in the Central Slovak area ("piatok"). The weak yers did not disappear in one syllable words, but the back yer changed to "a," and the front yer, to "ä". This change occurred already in the 10th century like in other West Slavic languages, contrary to the neighboring East Slavic area.