LEGO bushes,
also called bushings, are small elements put
on axles in order to maintain spacing between two or more
other elements. They can also prevent pieces from sliding off
of axles or keep axles in place. They come in three versions:
a half bush (half stud long), a bush (1 stud long), and a bush
with a long pin (3 studs long, including a 2-stud-long pin), all
of which are shown in Figure 4-6.