In the 10th century, Proto-Slavic ceased to exist, and Slovak began to emerge as an independent language. The most important early changes were the contraction, the loss and vocalization of yers and the denasalization of "ǫ" and "ę". These changes affected the word structure and phonemes. The loss of yers differentiated future Slovak, Czech and Polish from neighboring Slavic regions, and the denasalization differentiated Slovak and Czech from Polish. Slovak was not affected by old Polish dispalatization in the 10th century, causing differences between the two languages such as "žena" vs. Polish "żona" (a woman, a wife), "kvet" vs. Polish "kwiat", etc. It was also not affected by the old Czech syllabic depalatization before hard syllables, with differences such as "priateľ" vs. Czech sg. "přítel", pl. "přátelé" (a friend). Slovak preserved a difference between "dz"/"z" (from Proto-Slavic */dj/ */gtj/), i.e. "medźa" (medza, a boundary), "vítäź" (víťaz, an elite warrior, a winner) whereas both phonemes were transformed to ź in old Czech and dź in old Polish. Contrary to Czech, a vowel mutation from "à" to "e" did not occur in Slovak, i.e. "ulica" vs. Czech "ulice" (a street). The differences between Slovak and Czech like "ťažko"/"těžko", "cudzí"/"cizí" became stable later. Slovak developed only single "r" in contrast with Czech pairs "r"/"ř" and Polish "r"/"rz". Slovak evolved as an independent language already from the 10th century, and there is strong evidence against theories of its early or late formation from other languages.