Why does \cal{M}_n give “M>”?

Why does \cal{M}_n give “M>” in parts of my LaTeX file and works correctly in other parts?

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Instead of posting a “Thank you” in your question, you should thank egreg by upvoting his answer (with the upward pointing arrow to the left of it) and accepting it (by clicking on the checkmark). Additionally, you can comment his answer, too. – Qrrbrbirlbel Nov 24 '12 at 15:52

$\mathcal{C}_n$
${\cal C}_n$