You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Czech_language

Context:
While Old Czech had a basic alphabet from which a general set of orthographical correspondences was drawn, it did not have a standard orthography. It also contained a number of sound clusters which no longer exist; allowing ě (/jɛ/) after soft consonants, which has since shifted to e (/ɛ/), and allowing complex consonant clusters to be pronounced all at once rather than syllabically. A phonological phenomenon, Havlik's law (which began in Proto-Slavic and took various forms in other Slavic languages), appeared in Old Czech; counting backwards from the end of a clause, every odd-numbered yer was vocalized as a vowel, while the other yers disappeared.

Question:
Why aren't some sound clusters used in modern Czech?

no longer exist