When I first ran across the Stanley-Bostitch clamping level in Popular Science, I didn’t think much about it because it was pictured just sitting on a surface. Later I came across a picture of the level hanging off the bottom of a 2×4 and thought, “Hey, now that’s pretty cool!” It’s funny how much difference the choice of picture influences our perceptions of a product.
From the composite picture above you can tell that the clamping mechanism works two different ways. When clamping the level to 2x stock the whole clamping mechanism bites into the wood, while on wider stock smaller, yellow legs pop out of the level to hold onto the wood. Stanley-Bostitch doesn’t come right out and say it, but the clamps are probably spring-loaded. The take-away is that by clamping this level to the piece you are trying to position, your hands are free to maneuver it.
The center vial is magnified so you can see it easier and is bridged so you can use the level as a continuous straight-edge. The bubble vials are accurate to 0.00005in/in and Stanley-Bostitch even makes one of the vials rotate so you can duplicate angles other than the common plumb, level, and 45° found on other levels.
Clamping Box Beam Level [Stanley-Bostitch]