This system of vowel lengths is well preserved in Czech and to a lesser degree in Slovak. In the emerging modern Polish, however, the long vowels were shortened again but sometimes (depending on dialect) with a change in quality (the vowels tended to become higher). The latter changes came to be incorporated into the standard language only in the case of long "o" and the long nasal vowel, mostly for vowels located before voiced obstruents. The vowel shift may thus be presented as follows: