I can't explain it better than Wiki, so here you go "The Picatinny rail, alternatively known as the MIL-STD-1913 rail, STANAG 2324 rail, or simply "Tactical Rail" is a bracket used on some firearms in order to provide a standardized mounting platform, similar to the Weaver rail mount."
Source : http://en.wikipedia.org/wiki/Picatinny_rail
The Sketchup file contains a number of useful parts. There is a end with a hole for mounting with hardware. There is also a end that is simply a single end piece. Finally there is a center piece. You can stack any length of centers, and then finish them off with an end of your choice. Customize your length to what ever your need. I have provided an example that is two mounting hole ends with a single center. I have also made very short lengths just for pistol lasers, and such. So it seems to be very versatile, and yet also very strong, and prints well. There seems to be minimal deformity or failure on the ridges. I'm pleased with the product so far!