We receive questions from time to time about words rhythm and prism. These words are odd, because there is only one vowel phonogram in them, and yet in the way we pronounce them, it seems like there are two syllables. A vowel is what forms a syllable, linguistically speaking, so how can this be?
Linguistically, a vowel is a sound formed with the mouth open, unblocked by lips, tongue, or teeth. The sound can be sung or sustained with the mouth open. A consonant is a sound that is blocked by lips, tongue, or teeth; it cannot be sustained or sung with the mouth open. (Learn more: What is a vowel? What is a consonant?)
In all of these words, part of what is going on is that certain consonants have some vowel-like qualities. The sounds /l/, /m/, and /r/ can all be sustained like a vowel, and with /l/ and /r/ the lips and tongue are only partially blocking the sound. In some sound environments (run, meet, like, clam) they still function only as consonants, but sometimes - especially when they are adjacent to other consonant sounds, they end up kind of vowelly! And in some cases, this even adds a slight second syllable.
This happens with rhythm and prism, at least in the way most people pronounce them. The /TH/ and /s/ sounds before /m/ in these words plays a role too; because these sounds are formed in a very different place in the mouth than /m/, your mouth has to move quite a bit to say the /m/ sound after them. The result is a pronunciation that includes a slight schwa sound: /rĭthəm/, /prĭsəm/.
You can understand why by feeling it. Although it's possible to say these words without that schwa sound, it's pretty difficult to do. The words are much easier to pronounce if you add it in.
Spelling Rule 12.4
A customer emailed us to ask why the silent E rule "every syllable must have a written vowel" (Spelling Rule 12.4) isn't used in these words the way it is in table, wrinkle, and acre.
One could definitely argue it should have been! It would have been more consistent with words like "table" and "acre" if our English-speaking forebears had added an E in rhythm, prism, etc.
We don't know for sure why they didn't. This is just me speculating, but I am wondering if the vowel effect felt stronger and more noticeable to people in the /bl/ and /kl/ and /cr/ contexts than in /sm/ and /THm/. I know that if I try to pronounce the words as one syllable, I get closer to succeeding with rhythm than with table.