From the website: Cap is worked sideways in half double crochet rib to allow for stretch to fit a range of sizes. Hat can be made smaller (for preemies) by using thinner yarn and smaller hook.

Yarn: For smaller or preemie sizes, use sport-weight or baby yarn.

Hook size: Size H hook for worsted weight newborn size (for smaller or preemie sizes use size G hook with sport-weight or size F hook with baby yarn.) Pastel multi hat shown in Red Heart ‘Comfort’ worked on size J/10 (6 mm) hook due to thicker yarn.

Gauge: 4 sc = 1”, 16 hdc = 4”