Rider refers to the little extras a band receives from the promoter at a gig, usually in their dressing room. Typical riders include things like food and beverages, or a pre-arranged amount of money that the promoter will provide to a band to buy themselves a meal and some drinks. The rider should also include information about special dietary requirements or preferences of the band. The rider will usually contain specific information, such "the promoter will provide four hot vegetarian meals and assorted beverages."
Riders can go beyond the basic food and beverage components to include things like the number of dressing rooms a band requires, and even what kind of furniture the band would like in the rooms, what kind of flowers they want in the dressing rooms, and so on. If you're making demands like these on your rider, however, your concerts had better be extremely profitable. Otherwise, you may simply talk yourself out of a gig.
