V-Slot Rails
V-Slotted Rails are named based on their side in millimeters. For example a 2020 Rail is 20mm high, and 20mm wide.
These rails are some times called X Rails but shouldn't be confused with the X movement of printers, they also shouldn't be confused with V-Rails.
Many 3D Printer Gantries are made from V-Slotted Rails.
V-Slotted Rail Router Bit Specs.pdf