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