This piece is an example of extension lines. Extension lines are temporary lines normally used in drawing to aid in measurement from one object to another, or to keep things on the same plane. In this piece, the extension lines happen to be present in the finished work. This gives the work an unfinished and messy look, which can actually be quite interesting.

Artist: Alberto Giacometti
Title of Work: Reviews of Diego
Year Produced: 1953Medium: Oil
Source: http://www.surrealists.co.uk/viewPicture/103/

This is a good example of how to use extension lines in one point perspective. This was meant to be more of an example than an actual piece, so it leaves in all of the lines used. The lines here are used to help base things on the same plane as well as keeping them in the single perspective.

Source: http://www.bramhaa.com/draw-it-all-%E2%80%93-one-point-perspective-for-beginners-in-animation/691