Y - The end of the shaft to the first hole
Y2 - The last hole to the end of the shaft.
X – Spacing between the holes (Sometimes called distance on centers)
The Y-dimension applies to just about any linear rail, shaft or assembly with mounting holes. Below are shown several examples which are only a few of the products where the Y-dimension applies.
Knowing what the Y-dimension is necessary to ensure the
holes in the mating mounting surface are drilled at the correct
locations. When ordering a replacement part it is necessary to ensure
the parts will be the same. The Thomson standard is to have the
Y-dimension equal on both ends but special Y-dimensions are common.