Magnetic - horizontal and vertical levels. Small tool.
Very useful - fencing - posts.
A post level is, simply put, a spirit level that finds both level & plumb in a post or other type of 2x4. It has a simple, easy to use design: two-sided moulded plastic forming a 90° angle, a height of 5.5 inches and 3.5 inch width on each side, and three spirit level vials. The inside edge of the 90° prevents the material from pushing the post level away or out of whack.