The lower edge runs on a wheel - that part gets complicated,
actually. But they have big hydraulic cylinders extending and
retracting them, so even if they are a little sticky, it
doesn't mean much. - GWE

Terry King wrote:
Consider two pieces of square steel tubing, one 5", one 4",
both 1/4" wall thickness (these are hypothetical, not real).

..snip

This is the principle of how the telescoping boom on my knuckle
boom truck works.
Grant Erwin

Grant, That's an interesting idea; I wondered how those booms
were spaced internally.

How / what lubrication is used on those booms??