From Handknit Holidays:



"The colors and stitch pattern of this reversible scarf combine to create the look of stained glass.  A clever technique known as double knitting, in which two right-sided layers are created at once, forms the reversible stained-glass effect and results in a double-thick fabric that offers extra protection in bitter weather.  Two methods for double knitting are explained here:  two-stranded and slip and slide.  The two-stranded method is faster because you complete an entire row with each pass, but it takes a little longer to master.  The slip and slide method uses only one color at a time, and is recommended for beginners."



Pattern Clarification from Melanie Falick:

The pattern is correct as written, but here is a clarification to the note that appears at the beginning of the Two-Stranded Method.



When working this stitch pattern, with the exception of the first and last st, all sts are worked in pairs; either a k1, p1 pair or a p1, k1 pair. The k1, p1 pairs produce a knit st on both sides of the fabric; the p1, k1 pairs produce a contrasting purl st on both sides of the fabric. For all k1, p1 pairs, carry both colors in the same hand, taking both strands to the back when knitting and both strands to the front when purling; knit or purl with only the color indicated. For all p1, k1 pairs, separate the strands with the color to be purled in the front and the color to be knit in the back. Make sure the strands are not twisted.