A combination of the
words number and letter, in their literal sense. A lumber can be a letter, a number, or a combination of both.
Should be used in a conversation where the discussion can be about letters and numbers, and to avoid mistakes
like "What's the answer to number 'b'?", or "
Please reference section C... letter 14."
John: "Hey I'm on question 2 right now. Did you get number 'c'?"
Jane: "Number c? Sorry I have no idea what you are talking about."
John: "Oh, my
bad. Did you get lumber 2c?"
Jane: "Yes, yes I did."