Albert Flocon published a book of engravings some years ago, and Escher was particularly taken by the print "frere spirituel" in that book. From this he drew his inspiration for Knots, shown above. In order to create these prints, Escher first made a cardboard model of the print in the upper left. Look at the knot, then look away, and try to imagine such a knot. Imagine another sort of knot, and then try to draw it. As Escher said, "Oh dear, that knot! In the evening, I sometimes feel dizzy with the effort."
For an excellent lesson on the Mobius strip, a mathematical phenomenon related to Escher's knots, visit "Mobius Strip," located on an external web site.
All M. C. Escher works (c) Cordon Art B.V.-Baarn-the Netherlands. Used with permission.