The faceplate of a standard piece of equipment must measure 19.0 inches (482.6 mm) from edge to edge.
The holes along the vertical mounting rails are not spaced evenly. Instead, they follow a repeating 3-hole pattern every 1.75 inches. The standard dictates the following center-to-center distances between holes within a single 1U space: 0.625 inches (15.88 mm) Middle to Bottom Hole: 0.625 inches (15.88 mm) eia310d standard pdf
The "19-inch rack" name refers to the width of the equipment's front panel, not the opening of the rack itself. The standard breaks down horizontal dimensions as follows: 19.0 inches (482.6 mm) The faceplate of a standard piece of equipment
The standard defines a Rack Unit (U) as 1.75 inches (44.45 mm) of vertical space. This is the fundamental unit of measurement for rack-mountable gear, with devices built to occupy multiples like 1U, 2U, or 4U. Do you need help understanding for the 1U layout
Do you need help understanding for the 1U layout?