An abbreviation is a group of letters acting as a shortened form of a phrase such as CD, ATM, FBI, etc. Abbreviations that are pronounced letter by letter–such as ATM, AC–are called initialisms while abbreviations pronounced as words–such as ZIP, PIN–are called acronyms. The Gregg Reference Manual uses a great example with CT scan and CAT scan. CT is an initialism and CAT is an acronym. For ease here, we will call them all abbreviations. When using abbreviations, do not use a word that is part of the abbreviation with the abbreviation. For instance, do not use PIN number since PIN is the abbreviation for personal identification number, so you would be saying personal identification number number.
Once you have the abbreviation correct, how you treat the abbreviation in a sentence requires that you do not think of the phrase, but think of the abbreviation as its own new word. So if you’re trying to decide whether to use a or an with your abbreviation, think about how you would pronounce it. For example with PIN, it would be a PIN, and with ATM, it would be an ATM. See Listen to Choose an Appropriate Article to help you make that decision.
Other problems come when you are using an abbreviation for a phrase, i.e., TOS for Terms of Service or SAC for Second Amended Complaint. It can be confusing to determine whether to treat the abbreviation as singular or plural in deciding how to make it plural or possessive. Once you turn your phrase into an abbreviation, think of that abbreviation as a word and no longer the complete phrase. For example,
The Terms of Service’s provisions allow the prescribed activity.
The TOS’s provisions allow the prescribed activity.
In a Second Amended Complaint, you must redline the differences from the original Complaint.
In an SAC, you must redline the differences from the original Complaint.
Abbreviations are useful in legal writing (particularly with page limits), but try to keep it to something that makes sense and try not to turn every phrase into an abbreviation or your reader will have to have a road map in order to be able to make sense of your writing. While we are on that topic, when I’m working in a document with lots of defined terms (including abbreviations), I usually start a chart on my second screen or on a notepad so I can make sure the same phrase is abbreviated or capitalized consistently throughout the document as it is defined. It is the small things you can easily keep track of that will make all the difference in having a document that is correct and consistent.