Double letters
A doubled consonant is part of how a word is built. The trouble is that it can go missing where it belongs and turn up where it does not.
The rule
Doubled letters are part of how a word is built, not decoration. The reason behind a double varies from word to word — often an added ending, sometimes the word's history — so the double is learned along with the word.
The commonest reason is an added ending. After a short vowel and a single consonant, an ending that starts with a vowel usually doubles the consonant: stop, stopping; plan, planned; big, bigger. After a long vowel, or a vowel written with two letters, the consonant usually stays single: hope, hoping; stoop, stooping.
In longer words the stress decides it. The consonant usually doubles when the last syllable is stressed (begin, beginning; occur, occurred) and stays single when it is not (open, opening; visit, visited). American spelling follows that for a final L too (travel, traveled), where British spelling doubles it (travelled).
Many doubles in longer words mark a seam instead, where a prefix ends in the letter the next part begins with: il + legal, ir + regular, un + necessary, mis + spell.
The other direction
Not every consonant doubles. A double is there for a reason — often an added ending, sometimes the word's history — and where neither applies the letter stays single.
That is the miss in until, threshold and apology: words with the shape of a double and no double in them.
Where it stops holding
X is not doubled before an ending even after a short vowel (fixing, boxer), and neither are W and Y, which after a vowel are part of how the vowel is spelled (sewing, playing). Before an ending that starts with E, I or Y, a final C takes a K rather than a second C: picnic, picnicking; panic, panicked.
And a seam only explains a double if you can see the parts. In accommodate and embarrass nothing on the surface shows why the letters double, so the doubles are learned with the word.
Background
A double that marks an ending or a prefix is spelling recording a word's structure rather than its sound. How much of English spelling works that way, and what the research says about learning it, is on how people learn to spell.
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