It's a Series designator. If the Secretary of the Treasury changes, the BEP adds "A". If it changes again, they add "B" and so on...
They're worth a buck a piece. No letter just means its the original set of signatures for the series.
Well it's not series 1999. It's probably a 1988A note and if it's in very nice condition it is worth more than face value.
Spider said He originally asked about a 1999A Are you saying his '99 is really a '98, even though he's seen it and you haven't, or are you saying one of his other bills is a '98A, and he can't see the "A"
...there are no such things with FRN's dated 1999 (or 1998 at ALL) that have an A under them. They last did that with series 1988 when the treasurer Katherine Davalos Ortega left in 1989. I collect FRN's so believe me I know. Now though however the treasurer Rosario Marin left last December and a new one took over so word has it that 2003A's and 2004A's series should start showing up.
Sure is. If you wanna find out the exact year and month it was printed check out this site. http://www.uspapermoney.info/serials/f1988as.html