What is the meaning of "Alphabet of Lines"?

The Alphabet of Lines is a list of line symbols that engineers use in technical drawings to communicate specific shapes, sizes or surfaces. The lines and symbols used in the Alphabet of Lines are universal, which means engineers around the world use and understand them.

In the Alphabet of Lines, thick, dark lines represent the outline of an object, showing its visible surfaces and edges. Thinner construction lines show the placement of construction layout work. Short dash lines represent nonvisible surfaces. A phantom line is a long line followed by two short lines. This line shows the alternate position of a moving part.