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That is correct. It saves a lot of space, plus the expense of 4 linear bearings. You can see the detail on the second photo. The design utilizes inner slides, much like a T-nut, attached to the x axis ends with screws from the inside. The tolerances are very tight on these extrusions, so you can get very good sliding action. In another design, I would not be afraid to try it for the y axis.