Both dividers and trammel heads are used for the same purpose: to scribe circles or arcs on a workpiece and to transfer measurements. However, there are number of differences between the tools.
The measuring and marking capability of a divider is limited by the opening length of its legs.
On the other hand, the radius of circles that can be drawn using a pair of trammel points is only restricted by the length of the beam they are used with.
This means that trammel heads are used to draw larger circles than those that can be drawn with a compass or dividers.
The sharp point of a trammel can be replaced with a drawing instrument like a pencil. Some trammel heads can also hold sharp blades to score or cut a surface. Trammel heads can be adapted and used for a wide variety of layout tasks.
Dividers have fixed metal points and cannot be used with a pencil. The metal points of the tool are used to mark a surface.
