You can't definitively spell letters

Dave Allen tells this one. Working-class bloke walks into an interview, and the snooty interviewer asks him his name.

"Smif."

Interviewer gives him a dirty look, and says lingeringly, "How do you spell it?"

"S M I T H."

"Smith," the interviewer enunciates sourly, writing it down. "First name?"

"Arfur."

"How do you spell it?"

"A R T H U R."

"Arthur," the interviewer says very distinctly. "Age?"

"Fir'y-free."

"How do you spell it?"

"You don't spell it, it's a bleedin' number!"