UPDATE: A Solo rep has reached out and provided us with a new graphic. Turns out, while surprisingly accurate, those lines were not intended to measure spirits.
College students are familiar with Solo red cups. But they probably couldn't tell you what the lines mean.
The party cup actually doubles as a measuring cup — except for liquor instead of milk. The first line above the base is the one-ounce marker, or a serving of liquor. The second denotes five ounces, useful for measuring wine. And the third one signifies 12 ounces, perfect for beer (the cheaper, the better) — probably the only acceptable liquid for a Solo cup.