Oddly, there doesn’t seem to be an official standard, but it is so common that there’s usually not much variation between different vendors. Of course, aluminum extrusions aren’t made from modeling clay, but usually 6105-T5 aluminum. You push some material out through a die to make a shape. If you ever had a Play-Doh Fun Factory as a kid, then you know about extrusions. However, it is one thing to assemble a kit with some extrusions and another thing to design your own creations with the material. T-slot extrusions used to be somewhat mysterious, but today they are quite common thanks to their use in many 3D printers.
