Why do we say “un seul M” and not “une seule M” even though M is a “consonne”?

Question

When spelling names, for example “Bram”, to make clear that there's only one *M* at the end I hear French people say “Avec un seul *M* à la fin”. But “*M* est une consonne” therefore “un nom feminin”. So why is that?

Answer 1

Well,

That's the mystery of French!

the rule is that all letters (a, b,c,d,e,f,....) are indicated with the masculine article: un a, un m, un y, un s,... ou le b, le h, le i,...

Even if consonne et voyelle are feminine nouns. Une consonne, une voyelle, but un d, un p,...

Hope this helps,

Alain