Articles, prepositions, and conjunctions in acronyms

"Federal Bureau of Investigation" becomes FBI, instead of FBOI. Meanwhile, "Theory of Mind" becomes ToM or TOM, instead of TM.

Is there any rule regarding inclusion of articles, conjunctions, and prepositions in acronyms? What should I do with such phrases below, then?

• "And Then There Were None" - ATTWN or TTWN


• "The Lord of the Rings" - TLOTR, LOR, or LR


• "The Minority Report" - TMR or MR


• "A Thousand Splendid Suns" - ATSS or TSS

Answer

I am not aware of any specific formal rule governing when to include the initials of minor words in acronym. Informally, in my experience, initials of minor words are added when they reduce ambiguity, or make the acronym easier to pronounce or understand. And, as in your first paragraph, I often see minor words in lower case and more important words in upper case.