The dimension BTX is meant to stand for Boom tube (X). The fact that the dimension is measured horizontally is implied in the X. Input dimensions added to BikeCAD a long time ago tended to be fairly verbose. My intention was to use industry standard terms wherever possible and where no commonly accepted name existed, I'd just try to be as descriptive as possible. Overall, I'm satisfied with the result, but in a few cases, there are disagreements on the exact definition of certain industry standard terms and my use of long-winded descriptions for dimensions make it difficult when translating BikeCAD into other languages. What I've been doing more and more is adding little diagrams to the input dialog boxes. The diagram is meant to convey what the dimension does more clearly than could be done with a complicated dimension name. For this reason, I'm inclined to stick with BTX and BTY, but when I redo the diagram in my efforts to convert all images to scalable graphics, I'll make a point to emphasize the horizontal and vertical orientations of BTX and BTY. I will also plan to add a Boom tube (c-c) dimension.