What is the origin of the /k/ sound spelled "CH?" Is there a rule to know when to use ch for the k sound?
For the <ch> spelling of [k], see the preceding question on <ch> and <qu>. I don’t know of any firm rule for when to spell [k] with <ch>. If you can recognize a word as being from Greek, then any [k] sound will usually be spelled <ch>. Greek words have certain distinguishing spellings other than <ch>: for instance, <rh>, <ps>, and <y> spelling short and long <i> in the middle of words. Greek words are particularly common in scientific, medical, and other technical registers.
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