Note about the saddle in question 12(g) on Assignment 3:
There are some variations in perspective that are possible here, one being actually wrong (the x axis being rotated), the other two right (although there is a bit of rotation, it's not as serious!). Here are three "snapshots" of the saddle. The first two are OK, the third not really, since the x-axis has been rotated up and to the right (counterclockwise). (Notice it "looks" as if the negative x axis is pointing out at us.)