Rhythm is the showpiece of Greek spelling in English: rh- for rho, and y for upsilon, in a word of six letters with one vowel sound most speakers can identify. It comes from Greek rhythmos, "measured flow or movement," related to rhein, "to flow" — the same root as in hemorrhage and diarrhea.
But the English word is a respelling, not a fresh borrowing. Middle English had rime, from Old French, meaning measure, metre, and also agreement in end-sounds — what we now call rhyme. In the 1500s scholars produced rhythm as an attempted classical correction, and the two eventually split into different words. Rhyme then caught the h by association, which it has no right to at all. So one of these two words has a genuine Greek spelling and the other is wearing a borrowed costume, and it is not obvious from looking which is which.
rhythm (n.) 16c. spelling variant or attempted classical correction of Middle English rime "measure, meter, rhythm," also "agreement in end-sounds of words or metrical lines, rhyme; a rhyming poem" (12c.), from Old French rime "verse," from Latin rhythmus "movement in time," from Greek rhythmos "measured flow or movement, rhythm; proportion, symmetry; arrangement, order; form, shape, wise, manner; soul, disposition," related to rhein "to flow" (from PIE root *sreu- "to flow"). Compare rhyme . The word rhyme has no connection with the word rhythm, nor is rhyme necessary to accentual verse. Nevertheless, rhyme was usually present. On the other hand, in classical Greek metrical poetry, rhymes, if not accidental, were never an essential element of metrical verse structure. [Henry Osborn Taylor, "The Classical Heritage of the Middle Ages," 1911] The spelling fluctuated 16c.-17c., rithme and ri'me also being used. From 1550s as "metrical movement, movement in time characterized by equality of measures and alteration of stress and relaxation." By 1776 as "regular succession of beats or accents in music." The rhythm method in reference to birth control is attested from 1936. Rhythm and blues , U.S. music style, is from 1949 (first in Billboard magazine). also from 16c.
rhythm came in through Greek, and its spelling follows that family's conventions.
How that family writes these sounds
| /r/ at the start | rh-rhythm, rhetoric |
What gives it away on the page
| rh-, -rrh- | Transliterates ρ with its rough breathing.rhythm, rhetoric, hemorrhage, catarrh |