This pattern uses the DK yarn held double, as does the gauge.

The pamphlet I have has mixed up the gauge for this cap and the gauge for the balaclava. I have changed them over.

Measurements
Width round head
18, 20, 22 inches

Each pamphlet has a different set of recommended yarns. The following are from the Midgley and Catton version.

Materials
MIDGLEY AND CATTON Double Knittings
Homestead with Nylon or Tulip Triacetate Nylon
4, 5, 6 balls or hanks
Cable needle
Stitch holders.