"Vitruvian Man" by Leonardo da Vinci 1485-1490

A great example of balance in artwork.  A successful composition usually conveys the impression of balance, or equilibrium.  If you are a picture straightener, then you know how annoying it can be if an object is tilted and appears unbalanced.  The simplest form of balance is symmetry, in which there is an exact correspondence of parts on either side of a central axis.  In other words, the left side of a work is  a mirror image of the right side.
The human body is an example of symmetry (not exact), as shown here.  With the exception of the turned feet, if we draw a line down the middle, this is a mirror image.