What's the difference between a rocks glass and a highball glass?

A rocks glass is short and wide (8 to 12 oz), for spirit-forward drinks over ice. A highball is tall and narrow (10 to 16 oz), for spirit-and-mixer drinks with lots of carbonated mixer and ice.

Two of the most-used glasses in cocktail service, with different intended use: (1) Rocks glass (also called old fashioned, lowball, double old fashioned, tumbler) — short, heavy-bottomed, holds 8 to 12 oz. For: Old Fashioned, Negroni (on the rocks), Whiskey Sour, Sazerac, neat spirits with one large ice cube. Wide mouth for aromatics, heavy bottom for weight in the hand, sized for slow-sipped drinks. (2) Highball glass (also called Collins glass — the Collins is slightly taller and narrower) — tall, cylindrical, holds 10 to 16 oz. For: Gin and Tonic, Mojito, Tom Collins, Moscow Mule (when not in copper), Dark and Stormy, anything with substantial soda water or other carbonated mixer. Tall to hold the drink and a substantial column of ice, narrow to slow the loss of carbonation. If you only own one cocktail glass type, get rocks — they handle the most variety. Second purchase: highball, for all the spritzes and long mixed drinks. The Nick and Nora or coupe comes third, for served-up classics.