The logic behind the older usage 90's or 1990's is that an "e" has been omitted. The more recent custom, 90s, is actually ninetys, which is wrong. The apostrophe indicates the change from "y" to "ie" and this follows the practice of using the apostrophe for omitted letters..
Apostrophes
The logic behind the older usage 90's or 1990's is that an "e" has been omitted. The more recent custom, 90s, is actually ninetys, which is wrong. The apostrophe indicates the change from "y" to "ie" and this follows the practice of using the apostrophe for omitted letters..