Since their creation in the late 1920s a dual number system has been used for the names of Poul Henningsen lamps with their three-part shade system.
The numbers provide information concerning the size and relationship of the three lampshades to one another.
In the case of the PH 5/5 pendant lamp, the first 5 stands for the diameter of the upper, largest shade, which measures 5 decimeters, i.e. 50 cm.
The second 5 means that the two lower shades are proportioned 3:2:1 to the upper shade diameter.
The PH pendant lamps with three shades in this ratio are called "whole" PH lamps. They emit a particularly glare-free illumination and can therefore be hung higher in the room.