Spoken English is very flexible in its syllable structure. A vowel sound can constitute a syllable by itself—like the e in unequal (un·e·qual)—or can be preceded by up to three consonant sounds (as in strong or splint) and followed by up to four consonant sounds, as in tempts or sixths (which ends with the sounds k+s+th+s). But the English sound system is not without rules. Some combinations of consonant sounds, like p+k, can never occur within a syllable, and others can occur only at one end or the other. For example, the combination s+f can occur at the beginning of a syllable (as in sphere) but not at the end, while the reverse sequence f+s can occur at the end (as in laughs) but not at the beginning. The language does stretch occasionally to accommodate borrowings from other languages, as for words like schlep and tsar that can be said with an initial consonant cluster not native to English. And in a broad sense, even certain meaningful utterances composed exclusively of consonant sounds can be regarded as syllables. Examples include shh (urging silence) and psst (used to attract someone’s attention).
Breaking a written word into syllables—as in a dictionary entry, where the purpose is to clarify the structure of the word and assist in understanding and pronunciation, or in a book, for the purpose of end-of-line hyphenation—involves additional considerations. While based primarily on sound, the syllable divisions in spelled-out forms are also influenced by long-established spelling conventions, the etymology of the word, and the lack of an exact correspondence between spelling and pronunciation. For example, in writing, multisyllabic words with double consonants are conventionally divided between the consonants, even though the consonant is pronounced only once: sudden is divided as sud·den, though pronounced suddʹn. But the word adding—formed by combining the word add with the suffix -ing, is divided as add·ing to show its constituent parts. And a word like exact (pronounced igʹzakt) cannot be divided purely phonetically, because the letter x itself would have to be split; it is traditionally divided as ex·act. This means that even when divisions in the spelled form and the pronunciation do not match, they are both correct.