This pencil was invaluable for my graduate b-school exams. I used a combo .7 mm
tip for numeric problems and the .9 mm tip for the essay section of the
questions. The pencil is fool-proof if the correct sized lead is used and
although the mechanism may seem simplistic - it works. The only way to jam this
pencil is to try to force an oversized lead through a smaller diameter tip. The
tips can be swapped out with different barrels and the pencil can be customized
for different situations. This is the only pencil I am aware of that can be
customized in that manner.