Finally, the value of nearly any OpenGL parameter may be obtained by an appropariate 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 make it possible to implement various libraries, each without interfering with another's OpenGL usage.