Foreshortening is a technique used in perspective to create the illusion of an object receding strongly into the distance or background.
A familiar example of foreshortening would be when you look down a long straight road lined with trees, the two edges of the road appear to move towards each other and the trees look smaller the further away they are. Or if you're painting at a person lying on their back with their feet facing towards you, you'd paint their feet larger than their head to capture the illusion.
To see an example of foreshortening in a tall building, take a look at Monumental Glory by Dana Roach, in the Unexpected Angles Monthly Painting Project.
