EIA stands for the Electronic Industries Association. EIA-310 is a specification for the “standard rack”. This specification standardizes several important features of 19″ racks, such as the Rack Unit (RU or U), vertical hole spacing, horizontal hole spacing, rack opening and front panel width. The specification also sets tolerances on each of these dimensions. The current revision is EIA-310-D.
It’s important to note that there are certain rack characteristics such as hole type, mounting depth, front and rear space, and obstructions between front and rear rails that are not standardized.