The letter is an abbreviation, e.g. "G-man" as slang for a Federal Bureau of Investigation agent, arose as short for "Government Man".
Alphabetical order used as a counting system, e.g. Plan A, Plan B, etc.; alpha ray, beta ray, gamma ray, delta ray, epsilon ray
The shape of the letter, e.g. D-ring, F-clamp, G-clamp, H-block, H engine, O-ring, R-clip, U engine, V engine, Z-drive, a river delta
Other reasons, e.g. X-ray after "x the unknown" in algebra, because the discoverer did not know what they were.
