The z-index property determines the stack level of an HTML element. The stack level refers to the element's position on the Z axis (as opposed to the X axis or Y axis). A higher z-index value means the element will be closer to the top of the stacking order. This stacking order runs perpendicular ot the display, or viewport. In an HTML page, the natural stacking order (i.e. the order of elements on the Z axis) is determined by a number of factors. Below is a list showing the order that iterms fit into a stacking context, starting with the bottom of the stack.