I've just been hit with a big wave of nostalgia. Haven't used MS Office for over a decade, but I remember those green underlines. So if red was wrong spelling

I've just been hit with a big wave of nostalgia. Haven't used MS Office for over a decade, but I remember those green underlines. So if red was wrong spelling, and green was grammatical errors, then what were blue underlines used for? Or was there no blue at all?