"Take Charms and I shall drop Augusta a line reminding her that just because she failed her Charms O.W.L., the subject is not necessarily worthless."
Minerva McGonagall on the letter[src]

This letter was written by Minerva McGonagall to Augusta Longbottom in around September, 1996. Her purpose was to tell Augusta that Charms was not worthless, as Augusta claimed it was because she failed her Charms Ordinary Wizarding Levels and therefore recommended her grandson Neville Longbottom to take Transfiguration.