system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do street poles, like traffic lights and lamp posts often have two bolts thread BENEATH it raising it from the ground, before then having bolts on top holding it down?
gpt
They’re called leveling nuts. 

When they install light poles they first cast the foundation into the ground with the anchor bolts cast into the concrete. After the concrete is hard enough they come back with the light pole to bolt it on, but the top of the concrete isn’t going to be sufficiently flat - the unevenness of the surface would have the pole at an angle.

So they cast the bolts to stick a good distance out of the concrete then install the ~~bolts~~ *nuts* below, and carefully adjust them to get a level surface. Then they screw a second ~~bold~~ *nut* down hard on the first to lock it in place. Then they bring in the light pole, set it down, confirm it’s plumb and then bolt it down from on top.