Before a building is built, a bridge is constructed, or any product manufactured, a plan must be made. Architects, engineers, and designers must sketch the objects they wish to create. A drawing using special lines and symbols is then made that shows the exact size and shape of the object. These drawings are called mechanical drawings because they are done by hand with drafting instruments and tools. Drafters are the people who specialize in making mechanical drawings. Using these drawings as plans, builders and artisans can visualize and make the finished objects.
Drafters use special drawing sheets made of vellum (a type of paper), cloth, or film. They attach the drawing sheet to a drafting board, and draw, or draft, with special pencils and pens. Horizontal lines are drawn along the edge of a T-square; vertical and diagonal lines are drawn with triangles placed on the top edge of the T-square; and circles and arcs are drawn with a compass. Various shapes may also be drawn with templates.
Other instruments include the architect's scale, for measuring distances; dividers, for transferring measurements from one place to another on the drawing; French curves, for drawing irregular curves; the protractor, for measuring angles; the eraser; and drafting tape, for attaching the paper to the board.
A mechanical drawing may be multiview, usually showing two or three sides of an object. Each side (front, top, right side) is shown separately. The drafter adds dimensions so that a machinist, carpenter, or other builder can make or construct the product. A mechanical drawing may also be pictorial, showing the overall appearance of an object.
Today many designs are created through Computer Aided Drafting (CAD). When done on a computer, a mechanical drawing is made with the same standards and techniques used by an expert drafter.