The Jay hat is designed for serious warmth – worsted weight yarn worked in a double-knit pattern makes a very thick fabric perfect for trapping warm air. The geometric design keeps the two layers of fabric attached and makes the hat fully reversible.
Double-knitting is a technique that creates a fabric two layers thick. You will be working the inner layer and outer layer at the same time out of two different colors. To create fabric which is stockinette on both sides the outer stitch is knit first and then the inner stitch is purled.
All this means you will have twice as many stitches on your needles as normal for a hat of the given size. I recommend working on a 16 inch circular needle although magic loop or double pointed needles could also be used.