Just about to build a rack for some audio gear, and I'm looking for a suitable rack strip to use.
I noticed that there are rack strips with two or three holes per U. Why is this and which is used for what?
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Short answer: there are three holes because the specifications require it. Most rack units today have their mounting holes at the corners, but I have seen older multi-RU gear where the holes are placed in what would be the middle hole as well.
The reason two-hole-per-unit strips exist is probably because it works just fine for most gear. Keep in mind that the holes aren't equally spaced between units, so you usually wouldn't be able to move a rack unit one hole up for airflow, this is why you have vented rack panels.