One of the most common problems with selecting objects in complex drawings is that some objects obscure others. Even if the top object is partially transparent and you can see another object underneath, simply clicking it will still select the top object.
If the bounding box of the bottom object is smaller than that of the top one, you can 
-drag a rubber band around the small bottom object, and this will select it without selecting the top object. However, this is not always possible.
Inkscape offers a convenient way to select objects that are not at the top of the z-order: 
-click selects under the currently selected object. This means that the result of an -click depends on the current selection; if nothing is selected, or if you -click outside the current selection, the result is the same as if you simply clicked without .
If, however, you click a selected object and there are other objects underneath it, -click will deselect the selected object and instead select the object immediately beneath it. The next -click will select a still deeper object, and so on. When you reach the bottommost object at the click point, the next -click again selects the topmost one.
For example, suppose you have three stacked objects, numbered 1, 2, 3, from bottom to top. With nothing selected, -clicking them selects the topmost one, 3. The next -click selects 2, then 1, then 3 again, and so on.
-click differs from -click in the same way -click differs from a simple click: It adds to the selection or removes from it without unselecting it completely. So, in our 1, 2, 3 stack of objects, the first -click selects 3; after another -click you will have 2 and 3 selected; finally, one more -click adds the bottommost object to the selection as well—so, after three -clicks you will have all of 1, 2, and 3 selected.