From MapHist.com - Glossary of Terms for historical maps

Glossary of Terms
Standard parallel
By Pierre Joppen
Jul 7, 2004, 08:44

The line of latitude in a cylindrical or conic map projection where the surface of the projection either touches or intersects the globe. If contact with the globe is secant, the result is two standard parallels. These lines of latitude are significant since they define lines of zero distortion, along which there is true linear scale. Standard parallels are also referred to as standard lines. 

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