Phonologically, there is a unit in between the letter and the word: the syllable. It is on the basis of syllables that we determine whether words rhyme, and many languages place their accents based on syllable order. The syllable is broken into two parts: The Onset and the Rhyme, and the rhyme, in turn, breaks into the nucleus and the coda. The nucleus is the most.
The nucleus of a syllable is the head of the syllable, that is, you cannot have a syllable without a nucleus. The onset is the most sonorant element of the syllable, normally a vowel, but sonorant consonants, such as ‘n’ or ‘l’ are allowed, such as in button /bʌt/. sounds within a syllable tend to be arranged from least sonorant to most sonorant, up to the nucleus, and back down through the coda. Onsets are required in many languages, and possible in most languages, with only a few disallowing them. And finally, codas are required in only a few languages, and disallowed in many languages.
As well, languages have rules as to which consonant clusters are permitted, and these rules vary depending on whether the cluster is an onset, nucleus or coda.