Learn how to combine letters or shapes in Illustrator to give them an over-and-under look as though they were interwoven or interlaced, like in a Celtic knot. It's easy with the Divide Pathfinder and the Unite Shape Mode. You simply overlap the objects, divide them into separate objects where they overlap and then selectively reunite the pieces. Be sure to go full screen (the icon at the bottom right of the video) so you can see the controls well. (To exit full screen hit escape.)