A system of units
that falls somewhere between the English Engineering
system and the SI
system in terms of usability.
Units of force and length are basically the same as those in the English Engineering system, but mass is different.
As force must equal mass times acceleration, we can define only two of these units (or suffer the horror that is gc). Force and length are defined, and mass is derived.
The unit of mass is the slug. One slug is one lbf*sec^2/ft. Those units are pretty much nonsensical, but that's because mass is derived. One pound force will accelerate a mass of one slug at a rate of one foot/second/second. Since the acceleration due to gravity on the earth is 32.2 ft/sec^2, you can divide an object's weight in pounds by 32.2 and get its mass in slugs. Multiply by gc and you've got it in lbm (see English Engineering).
I actually quite like this system (at times), as I can use the units of feet and pounds, which is nice since that's what I still think in (I'm 6' tall, not 1.8m; etc.), without getting involved in the horror that is gc.