NiallBeag wrote:
Don't think of it as an exception -- it's more of a side-effect of pronunciation. Lenition is the "weakening" of a consonant, and originally occurred as a sort of lazy/easy pronunciation. But CCH isn't easy. For one thing, you've already made the C sound, so you're no saving any effort by saying CH. And CCH would be difficult to say in and of itself anyway.
The are various pronunciation-led rules in English that we don't tend to think of as exceptions : an vs a, or in- becoming im- (intractable vs impossible), and the sound im- appearing in words like "input".
An-fíor sin é.. My boss once asked me to proof-read a memo he wrote. In it was the phrase (in relevant part) “perform a LIA.” I said “Dude, that’s wrong. It needs to be “an LIA.” No, no no, “a” goes before consonants, “an” goes before vowels. I tried to explain that pronunciation sometimes supersedes spelling. He disagreed. Fine, boss, “h” is a consonant, do you say “a honor”? (He changed the memo.

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