It seems to me combining "a-"and "rhythmic" would intuitively be spelled "arhythmic".

Is there a rule or some other practical reason that it's spelled arrhythmic?

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    According to the Merriam-Webster, it's from the Greek "arrhythmos". – VampDuc Jul 29 '15 at 14:51
  • Arrhythmic: 1853, "without rhythm," in relation to musical sensibility, Modern Latin, from Greek arrhythmos. etymonline.com/index.php?term=arrhythmic – user66974 Jul 29 '15 at 14:57
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    Pirates. They say "Arr!" And they're not very rhythmic, even when singing "Yo, ho, ho!" – Hot Licks Jul 29 '15 at 15:21

Actually, "arhythmic" is recognized by many dictionaries as an alternate spelling (for example, Merriam Webster). As Henry notes, the Greek word is ἄρρυθμος "arrhythmos", so the spelling with one r does not come directly from Greek; it instead seems to derive from a re-combination in English of the elements "a-" and "rhythmic," exactly the way you intuitively wanted to spell the word.

The pattern underlying the main spelling, "arrhythmic," is that words starting with the letter rho (ρ) in Greek, transliterated in this position as "rh," tended to have the rho doubled when a prefix was added or when they were the second half of a compound word. Double rho (ρρ) is transliterated as "rrh." You can find more details and explanation on John Wells's phonetic blog: rh and rrh.

There are many Greek-based prefixes used in English. In general, after any of these prefixes ending with a vowel, a word starting with "rh" will have the spelling changed to "rrh."

Examples of doubing after other Greek prefixes: hyporrhythmic, Antirrhinum, Metarrhizium

For a word that already existed in Ancient Greek, this doubling exists in the original Greek spelling, and is generally brought over to the English spelling. Words newly coined in English from Greek components may follow this doubling rule, or they may not, depending on the intuitions of the person coining the word. For some words, both variants are used, as shown by the coexistence of "arrhythmic" and "arhythmic."

(Incidentally, another word that appears to vary in this way is Metarrhizium; while Merriam Webster gives the doubled spelling I was looking for to use as an example, you'll find that the Wikipedia article calls the genus Metarhizium with only one r.)

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    But is there a rough breathing over one of the ρ's in ἄρρυθμος? If not, then the question is really why is there an H in arrhythmia? If it's because there is one in the root, that's Greek and not English spelling. But since that decision has been made, right or wrong, that is now English spelling. – John Lawler Jul 29 '15 at 16:15
  • @JohnLawler: I believe there is, following the general rule that for doubled rho, the first rho always has a smooth breathing and the second a rough: ῤῥ. As far as I know, the sequence "rry" without an h cannot exist in the traditional transliteration system for Ancient Greek. – herisson Jul 29 '15 at 16:33
  • Any day I can blame my spelling deficiencies on a society that has been dead for thousands of years instead of blaming myself is a good day! – Cort Ammon Jul 29 '15 at 18:02

Although it originates in Greek as ἄρρυθμος, this kind of pattern is not uncommon in English.

A comparable example is irregular, which comes from Latin.

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    But if it's comparable it's not all that irregular, is it? – Hot Licks Jul 29 '15 at 15:22
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    Irregular isn't exactly comparable. It's a combination of ir and regular, so it makes sense that it would have two r's. – Nicole Jul 29 '15 at 16:23
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    @Nicole It's a combination of in- and regular with the in- prefix being assimilated into the word, the same way as in immobile. – Eric Jul 29 '15 at 18:08

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