The "mid-layer" is a warming layer worn between the base and shell layer. It's for instance the pullover you wear under your jacket. Mid-layer fabrics are very versatile in terms of look and feel. Some even come with windproof barriers or with a Durable Water Repellency (DWR) so they would double as a shell-layer.
The most common mid-layer garments are made of Fleece, Wool Interlock, Down and Frency Terry. Below is a basic guide to Merino wool Fleece and Merino Wool Interlock - the two fabrics we prefer the most when designing mid-layer garments.
Merino Wool Fleece
Merino wool fleece is of medium weight, soft and very warm. While the outer fabric side has a flat surface, the inner fabric side is fluffy. It's warmth is it's greatest strength and it will fair a lot better than a polyester fleece fabric. Merino wool fleece is suitable for pullovers, jackets, leggings and linings in garments.
Merino Wool Interlock
Merino wool interlock fabrics are of medium-weight, soft, firm and absorbent. While fleece is very warm, the interlock is warm but thinner, and therefore better at regulating body temperature. The Merino wool interlock fabric also functions well as a base-layer if it sits tighter against your body. If you're using it as a mid-layer, a looser fit is better.
If you look closely, you can see that both sides of the fabric look the same. With these fabrics, there's no need to pay attention to which side faces inwards or outwards.