"Now, pay attention!"
That's the basic meaning of N.B.--the abbreviated form of the Latin phrase "nota bene" (literally, "note well"). N.B. still appears in some forms of academic writing as a way of steering readers' attention toward something particularly important.
Two or three centuries ago, when classical Latin was widely taught in British and American schools, it wasn't unusual for Latin expressions to appear in English prose. For proof, pick up an American dollar bill and look at the Great Seal of the United States on the reverse (or "greenback") side.
There on the left, just above the floating eye and the unfinished pyramid, is the Latin phrase "Annuit Coeptis," loosely translated as "Providence has approved our undertaking." At the base of the pyramid is "MDCCLXXVI" (1776 in Roman numerals) and below that the motto "Novus Ordo Seclorum" ("a new order of the ages"). To the right, on the ribbon in the eagle's beak, is the country's first motto, "E Pluribus Unum," or "one out of many."
Now that's a lot of Latin for a buck! But keep in mind that the Great Seal was approved by Congress way back in 1782. And since 1956 the official motto of the U.S. has been "In God We Trust"--in English.
As the Romans used to say, "Tempora mutantur, nos et mutamur in illis." Times change, and we change with them.
Nowadays, with a few exceptions (such as A.D., a.m., and p.m.), abbreviations for Latin words and phrases have become rare in ordinary writing. And so our general advice regarding most Latin abbreviations (including e.g., etc., et al., and i.e.) is to avoid using them altogether. But when you must use them (say in footnotes, bibliographies, and technical lists), consider these guidelines on how to tell them apart and use them correctly.
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