Flannel lining refers to the process of lining the inside of a shoe or article of clothing with flannel cloth, for instance, a house slipper or a pair of chino pants. When a shoe or article of clothing is flannel lined, it will be able to provide that soft, warm touch of flannel you'd experience from a pair of flannel pajamas without looking so casual on the outside.
Let's picture a scenario. It's a cold winter's week day, and the first thing you do after getting out of bed is to pull on a pair of warm flannel pajamas pants. The warmth and cozy comfort they provide is much appreciated on a chilly morning, and you're already dreading the aspect of having to change out of them and into the chinos you'll be wearing to work. Obviously, wearing those flannel pajama pants to work isn't an option. But in a perfect universe, you'd still somehow have their insulation and comfort while retaining a professional appearance.
But as it turns out, you're already living in a perfect universe, thanks to a little something called flannel lining.
The chief benefits found in a piece of flannel-lined clothing will be the warmth and comfort that cotton flannel provides. Flannel attains its unique properties due to its "nap," a term that refers to the raised yarns that give it a soft, almost fuzzy texture. But those raised yarns contribute more than just a pleasing hand-feel: they also help to capture and retain heat, making flannel an excellent natural insulator.
In short, you can think of any piece of flannel-lined clothing as a garment that comes with its very own insulation.
For many, the plaid patterns (and associated bright colors) traditionally identified with flannel is one of the fabric's biggest selling points. While it's not technically true that all flannel fabrics feature plaid patterns, the idea of flannel fabric is so intertwined with plaid that the two are often considered to be one in the same.
So it can be somewhat of a disappointment to anyone who loves a plaid flannel that the colors and patterns of flannel lining will not be visible from the outside. While the invisibility of the plaid pattern comes with its own advantage of a more clean and professional appearance, those that wish the flannel's color and pattern could be exposed have a point.
However, the color and pattern of a flannel-lined pant can be exposed if you so choose it to be. Just as you might wish to roll up the cuffs of your jeans to expose the inside stitching, you can roll up the cuffs of a flannel lined pair of chinos to expose the flannel lining inside.
This simple act of rolling up an inch or two of pant leg changes everything about the look. Whereas before you could wear your flannel lined chinos with anything from a polo shirt for a classic preppy look to a fitted blouse during the weekday, exposing a bit of flannel lining suddenly provides a new look and aesthetic. Now the outfit has become a bit more rugged, and more amenable to t-shirts, fleece sweaters—and yes—flannel shirts.
You may also find that flannel-lined chinos worn with a cuff pairs with certain types of footwear particularly well. Pair it to moccasins or low boots as part of an autumnal outdoorsy look, or contrast it with white sneakers to elevate your street style. You can even consider wearing it with a classic pair of black ballet flats to add a patterned "pop" to an otherwise monochromatic look.