A common source of mispronunciations are phonotactic constraints related to the syllable. Syllables are units of organisation for sequences of speech sounds. Syllabic writing dates to the third millennium BC, so several hundred years before the first letters. Syllables are considered by phonologists to have a hierarchical structure. They are made up of a syllable nucleus (generally a vowel) which can be preceded or followed by consonants. The preceding consonants are called syllable onset, and ones that follow form the coda. The behaviour of speech segments may vary according to their position in the syllable. Languages vary in their syllabic structure.