Like many techniques in knitting, shadow knitting is deceptively simple. Shadow knitting relies on the difference in height between knit and purl stitches. The purl stitches from one row, block out the knit stitches from another row. This style of knitting is usually done in 1 light and dark color. When viewed straight-on, the piece of knitting looks like simple contrasting stripes. When viewed from an angle, a picture emerges from the knitted piece. Worsted weight yarn seems to work the best for this technique.
The pattern is worked on the wrong side of the item. The pattern stitches are knit on the wrong side, thus they look like purl stitches and stand out on the right side. The background stitches are purled on the wrong side. All right side rows are knit and the color is changed every 2 rows.
When you view charts for shadow knitting, they sometimes show additional color changes so you can see the picture that emerges from the knit and purl stitches. It is important to remember that you will not be changing colors except at the beginning of every other row when doing the actual knitting. The additional colors in the chart are only placed to help you visualize the picture that will emerge in the knitting.
This technique is simple and really fun. There are many free patterns on Ravelry for shadow knitting (also called illusion knitting) It’s also quite simple to design your own shadow knitting patterns. Vivan Hoxbro has a good book on this subject if you’re interested in exploring it further.