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 (it's a mirror-image of the correct graph), the other two right. Here are three "snapshots" of the saddle. The first two are OK, the third really not, since the x-axis has been rotated a half-circle. (Notice it "looks" as if the negative x axis is pointing out at us.) Since the y,z axes have not changed, this is a mirror-image - the axes form a "left-hand system", not a "right-hand" one.