Has a midpoint stop located 1 stud away from
one end (and therefore 4.5 studs away from the other);
has a half-stud-long section without grooves adjacent to
the stop on the longer side. Although its extra half stud
of length often makes it difficult to fit into a structure,
it has interesting applications. Figure 4-4 shows one
of them: The axle is inserted into a brick so that it protrudes by 1 stud on one side and its stop secures it from
sliding through the brick to the other side. This way, the
axle won’t slide out with the wheel (for example, while
making a tight turn), and the wheel can be driven using
the gear