This nifty cover was sent on February 26, 1863, from Washington D.C., to a Dr. J. R. Kelley, of Nantucket, Massachusetts, by his 20 year-old son, William P. Kelley(Kelly; He used two different spellings of his own, last name!), Company I, 20th Massachusetts Infantry! Sadly, on March 5, 1865, then-Sergeant Kelley would die in a Confederate P.O.W. camp in Wilmington, North Carolina.
To continue, as for rust red, Scott #65, outside of some slightly shorter perfs at its bottom, left corner, and two, missing/folded under perfs at its top, right corner, it's in pretty good condition. The cancel from the Washington D.C. P.O. is fair, but rather light/faded in some places.
Lastly, as for the envelope, itself, it's in pretty good shape for being over 161 years old. It's pretty clean. The back flap 90% intact. The envelope was opened at the right side, and has thus been reduced a bit. The right side is a bit jagged due to that. Overall, noted imperfections aside, this is still a really neat cover sent from a heroic Union soldier of the Company, "I," of the 20th Regiment, Massachusetts Infantry, William P. Kelley!