- This article is about a graphical user interface concept; for the space filling curve, see Z-order (curve)
Z-order is an ordering of overlapping two-dimensional objects, such as windows in a graphical user interface (GUI) or shapes in a vector graphics editor. One of the features of a typical GUI is that windows may overlap, so that one window hides part or all of another. When two windows overlap, their z-order determines which one appears on top of the other.
The term "z-order" refers to the order of objects along the Z-axis. In coordinate geometry, X typically refers to the horizontal axis, Y to the vertical axis, and Z to the axis perpendicular to the plane. Z represents the depth of the stack of overlapping windows. The z-order information thus specifies the front-to-back ordering of objects on the screen.
The method is not suitable for general use in 3D computer graphics, because objects cannot always be put into order in this way. For example, it is possible to arrange three rods in a triangle so that each overlaps the one immediately clockwise - none of the rods is "on top", because they are all overlapped by one of the others. This does not happen with GUI windows: they are always parallel to the screen, so all parts of the object are at the same depth. An even stricter restriction is found with layers in a graphics editor ; each layer completely overlaps those below it in the z-order.
Typically, users of a GUI can affect the z-order by selecting a window to be brought to the foreground (that is, above all other windows). Some window managers allow interaction with windows while they are not in the foreground, whereas others will bring a window to the front whenever they receive input from the user. It is also possible for special windows to be designated "always on top"; these are then fixed to the top of the z-order so that no normal window can overlap them.
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