M before P and B

"Antes de P y B se escribe M" (M must be written before P and B) is an easy-to-remember spelling rule of the Spanish language – and quite useful. It's actually a mnemonic Spanish speakers memorize as English speakers do with "i before e, except after c".

It means that you never write an N before P. It's always M. For example, campana, importante and emperador. Even composed words, whose original was spelled with an N, change their spelling when followed by a P. For example, cien+pies → ciempiés.

The same goes for B. You should use M without fail before it. For example, ambición, combinar and embarcación. Now, you must notice that this is good for B, but not for V. If the word is spelled with a V, then you should place an N before it. For example, convento, envío and invitación.

There are no exceptions to this rule. Of course, foreign words don't count.