In addition, Backgrounds can store script and so can the Stack itself. This means that information stored in global variables or handlers at the stack level can be recalled at any other level. Likewise information stored in the Background level can be used at the card level or with fields or buttons. Handlers are a key element of HyperCard programming. A handler is a command.
Handlers come in pairs.
Handlers must begin and end all active HyperCard script. When a button has been created the default handlers ON MouseUp and End MouseUp are automatically created. Handlers can be any keyboard command or mouse command. MouseUp is activated when the mouse has been depressed and let up. User created handlers are not based on actual computer events.