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A schematic diagram represents the elements of a system using abstract, graphic symbols rather than realistic pictures. A schematic usually omits all details that are not relevant to the information the schematic is intended to convey, and may add unrealistic elements that aid comprehension. For example, a subway map intended for riders may represent a subway station with a dot; the dot doesn't resemble the actual station at all but gives the viewer information without unnecessary visual clutter. A schematic diagram of a chemical process uses symbols to represent the vessels, piping, valves, pumps, and other equipment of the system, emphasizing their interconnection paths and suppressing physical details. In an electronic circuit diagram, the layout of the symbols may not resemble the layout in the physical circuit. In the schematic diagram, the symbolic elements are arranged to be more easily interpreted by the viewer.
A semi-schematic diagram combines some of the abstraction of a purely schematic diagram with other elements displayed as realistically as possible, for various reasons. It is a compromise between a purely abstract diagram (e.g. the schematic of the Washington Metro) and an exclusively realistic representation (e.g. the corresponding aerial view of Washington).
Electrical and electronic industry 
In the electrical industry, a schematic diagram is often used to describe the design of equipment. Schematic diagrams are often used for the maintenance and repair of electronic and electromechanical systems. Original schematics were done by hand, using standardized templates or pre-printed adhesive symbols, but today Electrical CAD software is often used.
In electronic design automation, until the 1980s schematics were virtually the only formal representation for circuits. More recently, with the progress of computer technology, other representations were introduced and specialized computer languages were developed, since with the explosive growth of the complexity of electronic circuits, traditional schematics are becoming less practical. For example, hardware description languages are indispensable for modern digital circuit design.
Schematics for electronic circuits are prepared by designers using EDA (Electronic Design Automation) tools called schematic capture tools or schematic entry tools. These tools go beyond simple drawing of devices and connections. Usually they are integrated into the whole IC design flow and linked to other EDA tools for verification and simulation of the circuit under design.
In electric power systems design, a schematic drawing called a one-line diagram is frequently used to represent substations, distribution systems or even whole electrical power grids. These diagrams simplify and compress the details that would be repeated on each phase of a three-phase system, showing only one element instead of three. Electrical diagrams for switchgear often have common device functions designate by standard function numbers.
Schematics in repair manuals 
Schematic diagrams are used extensively in repair manuals to help users understand the interconnections of parts, and to provide graphical instruction to assist in dismantling and rebuilding mechanical assemblies. Many automotive and motorcycle repair manuals devote a significant number of pages to schematic diagrams.
See also 
- Thomas E. French, Charles J. Vierck Engineering Drawing and Graphic Technology Eleventh Edition, McGraw Hill, 1975 ISBN 0-07-022157-X pp. 621-624
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