The Pitch Line of a wheel, is the circle upon which the pitch is measured, and it is the circumference by which the diameter, or the velocity of the wheel, is measured.

The Pitch, is the arc of the circle of the pitch line, and is determined by the number of the teeth in the wheel.

The True Pitch (Chordial), or that by which the dimensions of the tooth of a wheel are alone determined, is a straight line drawn from the centres of two contiguous teeth upon the pitch line.

The Line of Centres, is the line between the centres of two wheels.

The Radius of a wheel, is the semi-diameter running to the periphery of a tooth. The Pitch Radius, is the semi-diameter running to the pitch line.

The Length of a Tooth, is the distance from its base to its extremity.

The Breadth of a Tooth, is the length of the face of wheel.

A Cog Wheel, is the general term for a wheel having a number of cogs or teeth set upon or radiating from its circumference.

A Mortise Wheel, is a wheel constructed for the reception of teeth or cogs, which are fitted into recesses or sockets upon the face of the wheel.

Plate Wheels, are wheels without arms.

A Rack, is a series of teeth set in a plane.

A Sector, is a wheel which reciprocates without forming a full revolution.

A Spur Wheel, is a wheel having its teeth perpendicular to its axis.

A Bevel Wheel, is a wheel having its teeth at an angle with its axis.

A Crown Wheel, is a wheel having its teeth at a right angle with its axis.

A Mitre Wheel, is a wheel having its teeth at an angle of 45° with its axis.

A Face Wheel, is a wheel having its teeth set upon one of its sides.

An Annular or Internal Wheel, is a wheel having its teeth convergent to its centre.