GDPE diagrams lines 03

A line is classically defined as a connection between two points. I say "classically" because that is the way that classic geometry texts – specifically Euclid’s Thirteen Books of the Elements – defined it in definitions II and III:

Euclid's Definitions, from The First Six Books of the Elements of Euclid, by Oliver Byrne (1847)

Euclid was really defining a straight line. But artists and designers know that lines come in many different configurations: they can be curvy or wavy; they can be jagged and rough; they can demarcate a border between things, thereby separating them. But they can also connect things together. They can imply movement, draw our attention to things, or obscure things. Lines can be angry, sad, mysterious, direct, or implied by the alignment of other elements (a.k.a. implied lines).