We describe a method for inferring constraints that are desirable for a given (rough) drawing and then modifying the drawing to satisfy the constraints wherever possible. The method has been implemented as part of an online graphics editor running under the UNIX™ operating system and it has undergone modifications in response to user input. Although the framework we discuss is general, the current implementation is polygon-oriented. The relations examined are: approximate equality of the slope or length of sides, collinearity of sides, and vertical and horizontal alignment of points.