This is an attempt to standardize rod spacing on mendel style 3D printers that are adopting a vertical orientation for the X axis smooth rods. Some examples include RepRap 1x2, Mixtape Mendel, Emaker Huxley, and a rumored Prusa variant from Russia. The standard will be informal. Those wishing to design conforming bots, X carriage designs, X axis assemblies, etc. can use this as a starting point. Hopefully many of the designs will inter-operate, allowing easier cross pollination.

The image was done by RepRap user Emanuel

Current thought is to have the vertical smooth rods have a separation of 70 mm center to center. For those wishing to join in the debate please use the RepRap forum topic here. "http://forums.reprap.org/read.php?2,95828"

Here is the description page on the reprap wiki

http://reprap.org/wiki/Vertical_X_Axis_Standard