Those cups became very popular in the 1970s, particularly amongst younger people living at home or going to college. If you and your buddies from college were having a big party, you could buy a sleeve of cups and a beer keg a have a good time. I remember going to parties like that. The law said people were not allowed to sell alcohol without a license, so to cover the booze costs, the hosts would sell cups at the door that people could drink from.

I sincerely think that Red Solo Cups beat out other disposable cups are because of the lines. They are actually measurements that you can use to measure mixed drinks if you're not drinking beer or wine.