Standard Parallels are lines at which there is no distortion in the map projection. Standard parallels are used in both conical and cylindrical projections. They theoretically touch the earth’s surface. These parallels contain no distortion, meaning that when the new representation is created there will be error on the projection except on that line.  To measure distance on a map, it is important to select a projection that accurately represents distance, an equidistant projection. These projections maintain a constant distance from the center point. One way to accomplish this is to use standard parallels. Standard parallels are parallel lines that are drawn on a reference globe that will maintain a scale factor of 1.0. Once standard parallels are drawn, all distances measured from within these lines will be accurate.
In a tangent conic or cylindrical projection, there is only one standard parallel. However, to reduce distortion, a secant projection may be used, which contains two standard parallels.  At each standard parallel, the projection shows no distortion.  Lines of true scale are called standard parallels for conic projections, and central meridians for cylindrical projections. 
- Demers, M., Fundamentals of Geographic Information Systems. John Wiley and Sons, Inc. 2009. pg 65. (p. 118, 242). New York, NY: The Guilford Press.
- Standard Parallel | Definition - Esri Support GIS Dictionary, support.esri.com/en/other-resources/gis-dictionary/term/standard%20parallel.
- Lo, C.P. and Albert K. W. Yeung (2002). Concepts and Techniques of Geographic Information Systems, Upper Saddle River NJ: Pearson Prentice Hall
Demers, M., Fundamentals of Geographic Information Systems. John Wiley and Sons, Inc. 2009. pg 65.