When drawing a graph, the programmer usually wishes to specify the coordinates of the points to be plotted in terms of the values of the variables involved. These coordinates are called <#819#>world coordinates<#819#>, and may have any floating-point value representable by the computer. The <#820#>window<#820#> refers to the rectangular region of world-coordinate space which is to be graphed. This window is mapped onto a rectangular region of the <#821#>view surface<#821#>, which is (a portion) of the screen or sheet of paper in the output device. This physical region onto which the window is mapped is called the <#822#>viewport<#822#>. Before a graph can be drawn, the program must define both the window and the viewport by calling appropriate routines in PLPLOT.