Jake, Looking at your Midpoint constraint image, it looks like you are in a Part workspace sketch. (2D drawing sketches don't allow reference lines as shown on your image.) What you have there is possible and may just need to apply the constraints in a different order. I have found that sometimes Alibre gets confused and changing the order of constraints "fixes" things. Here is a copy of your sketch, fully defined with the constraints you wanted. Note that I started the rectangle on the Origin. If it were not I would have Anchored an end point or constrained it to some other geometry to fully define the sketch.