Overlapping in art is the placement of objects over one another in order to create the illusion of depth. Painting is a two-dimensional artistic expression. It has length and width but no depth. It is necessary, therefore, for artists to provide viewers with some sort of perspective in establishing size and distance in paintings. This is where overlapping come into play.
If everything in a painting was of the same basic size, without overlapping there would be no way for viewers to distinguish small but important details, such as who or what is closest to or farthest from the viewers. Overlapping turns paintings into windows of sorts by creating the illusion that there is an entire world inside the canvas and that viewers are merely getting a glimpse of it. Overlapping was an aspect of works of an art form that emerged just before the middle of the 20th century called abstract expressionism. Many abstract expressionist paintings are simply a series of overlapping lines or shapes. Overlapping can also be used to blur the lines of where one thing starts and another begins. Pablo Picasso's Three Musicians is an excellent example of this. The famous cubist painting appears to be comprised of paper cutouts positioned to create the illusion that the three musicians merge.