A good set of flannel sheets should be so warm and inviting that it’s hard to leave them on a cold morning—if you don’t want to stay in bed all day, they’re not doing their job.

Hence, when the days get cold, flannel-lined jeans will make it all the more bearable. Flannel-lined jeans are the best of both worlds: Rugged exterior denim united with cozy interior flannel.

Flannel is most often affiliated with its workwear roots, originating in the 17th century as a hardy garment for Welsh farmers. Defined by its soft yet sturdy hand—achieved by carding the yarn ...