Compasses have been used for many years for navigation, technical diagrams and drawing. The tool is designed to allow the user to draw accurate circles of varying diameters. Navigation compasses were used on sail boats to determine course direction and position based on the wind. These were different from the other use of the word compass, which indicates which direction is north.
The most basic form of a compass involves two arms connected at the top by a hinge allowing them to pivot. On one arm there is a sharp point which is placed at the center of the circle and on the other a pen or pencil to actually draw the circle. The writing instrument can either be built into the compass arm itself or attached independently by the user.
A bow compass follows the same basic design as the drawing compass, but has a screw built in between the arms which can be adjusted by turning a wheel in the middle. This means it is able to hold the arc of the circle more accurately than the manually changed drawing compass. It is called a bow compass as the arms are manufactured in a slight arc to allow it to be adjusted.
This version is a combination of both the drawing and bow compass. The arms are pivot in the middle and cross each other, similar to a pair of scissors. This design uses a screw mechanism to hold it in place like the bow compass to maintain accuracy.
The drop compass only has one pivoting arm which holds the writing instrument, while the other is fixed in position. The compass is adjusted using a screw arrangement similar to the bow compass.
The two arms are mounted on a straight edge ruler and are fixed by screws at the top. This means adjusting the compass is very accurate and it can also draw larger circle than the other versions.