There are 3 basic knitting operations for fabric used in base layers:

Open Width - The fabric is knitted on a wide width machine which is usually 75cm in diameter. Once knitted, the fabric is slit open and laid flat on a cutting table. The advantage of this type of knitting is that you maximise the yield but the disadvantage is that the garment is not a neat fit and is more like a loose T-Shirt with side seams.

Double and Single Jersey - These can be knitted open width but for the optimal base layer thermal the make should be ‘body diameter’ (fabric is knitted tubular into different widths to cover the normal spread of adult waist and chest measurements). Base layers made using body diameter knitted fabric will have no side seams and therefore optimise fit and performance. Body diameter machines are highly expensive, so only the real thermal-wear specialists will outlay this expense for knitting machinery.

Rib Knit - This is the most basic of knits and can be either open width or body diameter. Many brands will choose this method of knitting because the machinery is cheap (approximately 90% less than the body diameter knitting machines) and thus lends itself to cottage style manufacturing.