Object and Animation Basics - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 51

Figure 3.24 A low-resolution terrain with proxy trees and a character.

header control which layers are displayed.
In Figure 3.25 , the layer buttons are
numbered. The scheme is simple: the top
row represents layers 1-10; the bottom,
11-20. LMB clicking on a layer button
works just like all other button sets in
Blender: it clears the current “selection”
and replaces it with the one clicked. To
display the contents of several layers at
once, you build a selection with

Shift-LMB.

In the case of our example, the trees
have been assigned to layer 2. Note on Figure 3.25 The layer buttons, numbered.