On most equipment racks you will find the following pattern of holes:
** * ** * ** * ** * **
No, it was not drilled by a drunken woodpecker.
See the holes with the wider spaces between them? Good… IGNORE THEM! They do not exist! You never saw them. Fnord.
See the closely spaced holes? Imagine they’re the dots on a double 1 domino. This means there’s a center line between them. See attached photograph. The center line will be the edge of each piece of equipment to be mounted.
Why are the center holes there? Okay… Fine, I’ll admit to their existence. Begrudgingly…
Some oddball accessories like cover blanks and cable lacing bars may be screwed in here. In the case of cover blanks it’s usually on weird ones that have a single hole in each end of a 1U* high blank. If you install a lacing bar this way it will be in the middle of one rack unit space – handy for the rear rails behind a patch panel.
Otherwise, do not use them unless you really have good reason to.
If you do mess this up…. Well, look at the wonderful mess in the photograph. Huuurrrrrgghhhhhhhhhbbllll.