Happy birthday to the 19" equipment rack, which turns 100 next month. From the July, 1923, Bell Systems Technical Journal: "All panels are to be of a uniform length, designed to mount on vertical supports spaced 19>< inches between centers. The height of the different panels will vary, according to the amount of apparatus in each unit, but this vertical dimension is in all cases to be a whole multiple of 1-3/4 inches."(p139 at https://archive.org/details/bstj2-3-112/page/n27/mode/2up )