Why do initials like PBJ (peanut butter and jelly) and NIB (new in the box) not include the first letter of each word?

The reason initials like PBJ (peanut butter and jelly) and NIB (new in the box) do not include the first letter of each word is because some of the words like "and" and "in" are regarded as insignificant.

Explanation:

Other less significant words like "of"  and "the" are usually dropped from initials.  The chosen letters are the key phrases, whose omission can render the initials unintelligible.  But grammatically, there are no hard and fast rules concerning the initials to use and the ones to omit.  It is a matter of conventions, established through long-term or first-term usages.

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