The main spelling (~100%)
⟨r⟩ ~99%
The letter ⟨r⟩ is the standard spelling for /r/.
⟨rr⟩
Doubled ⟨rr⟩ appears between vowels when the preceding vowel is short.
Unusual spellings (<1%)
⟨wr⟩
The ⟨wr⟩ combination has a silent ⟨w⟩. This spelling comes from Old English, when the /w/ was actually pronounced.
Historical note: In Old English, "write" was pronounced with both /w/ and /r/ sounds: /ˈwriːtan/. The /w/ disappeared from pronunciation but the spelling remained.
Two ways to see this: You can think of ⟨wr⟩ as an unusual spelling of /r/, or as a silent ⟨w⟩ before ⟨r⟩ — see the
silent W page for the full list. Both are valid.
⟨rh⟩
The ⟨rh⟩ combination has a silent ⟨h⟩. This spelling marks words borrowed from Greek.
Greek origins: Words with ⟨rh⟩ came from Greek, where the letter rho (ρ) at the start of a word was written with a "rough breathing" mark, which Latin transcribed as ⟨rh⟩.