A letter sign is used before names, such as Al and Ab, and words that have the same spelling as short-form words when they fall at the beginning of a sentence, quotation, or line of poetry, or in fully capitalized text. A letter sign is also used for letter groupings, such as abbreviations, when they could be confused with short-form words. Examples:
Al said to his sister, "Ag, come here!"
TUESDAY: THE SMITH AND IMM FAMILIES' REUNION
Can I borrow that CD-ROM?
It should be kept in mind that the purpose of the letter sign is to avoid confusion of letters or letter combinations with numbers, contractions, or short-form words. If the context is perfectly clear and no such confusion is possible, the letter sign is not used. Examples:
the XYZ Affair
Take the frontage road (KK) home.
Please take Uncle Al home now.
SD and JY connect at W.
We have a fine PD.
Some police departments aren't so good, but we have a fine PD.
John is in ag school.
John is interested in animal husbandry and will be going to ag school.