[admin] on page 2 a diagnosis, explanation, and alternate modeling approach is explained. This behavior is expected. The attached frame has a diagonal that measures the same as the diagonal dimension it is to align with. It will not align. I know why it will not, I set it up that way on purpose as an example. There is a difference in the actual measurement between the diagonal bar and the diagonal of the frame. But as far as you can tell by using the tools available, there is no difference. The difference is a couple of decimal places over past what is displayed. Now, all of us here know perfectly well that if the parts were machined exactly as they are represented here, the assembly would go together, and no QC department this side of the NIST would discover an error. And there is no reason to worry about "all the rest of the assembly being 'off' on account of this error", the error is unmeasurable by any commercial QC department. So, it seems pretty silly for Alibre to refuse to align these parts. Yet it does, and there is nothing to be done but to "cheat" the alignment (or use Limits). It seems eminently practical to allow a settable number of decimal places inaccuracy or error band to be allowed in an alignment if desired. The way I see it working is that the alignment would attempt to put it in exactly, and would do that if possible. The difference is that if that is not possible, but the error between the exact and the possible positions is within the error band, then the alignment would proceed without error. We could argue about whether the position of the resulting alignment should be considered as its true position, or as the position of the fixed axis (etc) to which it was being constrained. I see decent arguments for both, and I suppose it would be best to do whatever is the easiest to do, since we have already decided that the error is insignificant by allowing the error band to begin with. This idea differs from the limits option, because limits is dynamic, and there is even a warning about using it this way, because of the resources it uses. Limits does work here, of course. If you set the limits parameters to 0.000001", the parts will align. But you do use unnecessary resources. I see this as static, there is no good reason why it should be dynamic, it can be aligned and fixed there without any capability of movement beyond what is allowed for any other use of the same constraint. The only case for re-evaluation of the constraint would be if another constraint was added that might shift the position of the diagonal bar in this example. If that shift were still within the allowed error, then no error should be thrown. Alternately, the added constraint might have to be made with the use of the error band on it's alignment, which would not require any re-evaluation. That is probably the simpler and more reasonable approach, as it removes the apparent need for Alibre to "read minds and decide what you really want". I see this mainly in the coaxial alignment case, but there is no reason that it should not apply to others. And I see no reason why a small error of angle between the axes of the two holes (or two planes, lines, etc) should not be included as a settable error band in the alignment. There may be errors of distance, or of angle. I would not hold out for that angle error if it meant the difference between getting the option or not getting it. However, it seems entirely in-keeping with the idea, and desirable if possible to do.