In telecommunication, computer network or audio/video industry, 19-inch rack chassis cabinet is widely used.19-inch means the width of the equipment front panel, and not the distance between holes.19 inch is about 48.26cm or less than half one meter. As to how tall the chassis is , it is measured in Rack Unit (RU).Generally, one chassis could hold multiple RUs. Here, 1 RU equals to1.75 inch, which is about 44.45 mm. 1.75 inch is the not height of front panel. The fact is the height of the front panel is 1/32 inch less --- about (44.45 - 0.79) = 43.66 mm. Such design will give some spaces between RUs in the rack.
When buying chassis based switch/routers or servers, good to buy those standard chassis. When looking into
the data sheets or production information next time, you know what those terminology means.
23 inch or 24 in rack also exist in industry, but not as popular as 19 inch rack chassis.