The value of nearly any OpenGL parameter may be obtained by an appropriate get command. There is also a stack of parameter values that may be pushed and popped. For stacking purposes, all parameters are divided into 21 functional groups; any combination of these groups may be pushed onto the attribute stack in one operation (a pop operation automatically restores only those values that were last pushed). The get commands and parameter stacks are required so that various libraries may make use of OpenGL efficiently without interfering with one another.