A plum is the fruit, and figuratively something desirable — a plum job. Plumb with a B means exactly vertical, or to measure depth: plumb the depths. Both come from Latin, and the B belongs to the lead weight a plumb line uses.
The core difference
A fruit and a vertical measurement, one letter apart.
- plum — the fruit; something choice: a plum role.
- plumb — exactly vertical; to sound the depth: the wall is not plumb, plumb the depths.
The lead connection
Plumb is from Latin plumbum, lead — the metal. A plumb line is a cord with a lead weight, which hangs exactly vertical. That is also where plumber comes from: originally a worker in lead pipes. The chemical symbol for lead, Pb, is from the same word.
The adverb
Plumb also works as an intensifier meaning completely — plumb crazy, plumb forgot — chiefly in American usage. That takes the B as well.
Frequently asked questions
Which spelling means vertical?
Plumb, with the B — from Latin plumbum, lead.
Why is a plumber called that?
From the same root — originally a worker in lead pipes.
What does "a plum job" mean?
A desirable one — the figurative sense of the fruit, spelled without the B.