Example of Letter

The word 'letter' has taken on several meanings over time, all of which are quite different.
The English usage has been affected by the French 'lettre' in old usage, which expanded its meaning.

Letter- individual element of an alphabet.
Letter- Written message in the form of a personal letter.
Letter of the law- Literal interpretation of law.
Letter of marque- Official authorization to conduct acts of piracy

Examples of Letter:

The Greek Letter A was derived from the shape of formations of birds.
A letter was received from the claimant demanding action be taken.
The strict letter of the law would not be appropriate in this case.
A letter of marque was an old form of recruiting naval mercenaries.
https://en.wikipedia.org/wiki/Letter_%28message%29
https://simple.wikipedia.org/wiki/Letter
https://www.career.vt.edu/JobSearchGuide/CoverLetterSamples.html

Image Example of Letter:

A famous letter from Albert Einstein and Leo Szilárd to U.S. President Franklin Roosevelt suggesting an atomic bomb project.