I am writing a letter to a friend who has just been made a Captain in
the British Army. How do I address him on the envelope?

Captain xxxxx

Cpt

This is from Debrett's Correct Form:
"Naval, Military, and Air Force Officers
Professional rank should always precede any titles, e.g., "Adm (the Rt
Hon) the Earl of_____","Gen the (Right Hon) Lord_____", "Air-Marshal
Sir", but Lieutenants in the Army, Flying Officers and Pilot Officers
in the Air Force are addressed by their social and not their
professional rank, e.g. "The Hon [Benjamin]_____, Irish Guards",
"George_____, Esq, 11th Hussars", or "[William]_____, Esq, RAF"."
http://www.debretts.co.uk/etiquette/correct_forms_of_address.html#officers

"Army
The appropriate letters that signify a Regiment or Corps may be placed
after the name for officers on the active list of and below the rank
of Lieutenant-Colonel, but are often omitted in social correspondence.
These letters are not used for retired officers.
Corps have letter abbreviations (e.g. RE, RAMC, RAOC, RAPC). Most
regiments are written in full."
http://www.debretts.co.uk/etiquette/post_nominals.html#ArmedForces

Looks as if it should be Dear Captain ****,
Unless he's a good friend and it is a social letter.
answerfinder-ga

Thanks guys. Answerfinder, that was just what I was looking for. I
love the fact that Debretts assumes initially that one is writing to
an Honourable!

If I was addressing a letter to a friend who was a Captain in the
British Army, I would address it Dear Cpt Pugwash on the envelope, &
Hey Pugs in the letter.