Radius following 

Scribes lines with a tungsten carbide replaceable blade

follows and scribes round curved surfaces.

comes with a scribe for curves and also takes pens and pencils.

Single Blade

Tungsten carbide replaceable blade, accurate scale with 

ability to measure hole depths etc.

Double Blade

twin blades for marking parallel lines

scale for accurate setting, one of the blades can retract entirely for 

use for marking single lines. Can be used as a depth gauge,

blades are rotatable and replaceable.