Longjohns

Longjohns and thermal legwear, the layer worn closest to the skin

Longjohns and their modern relatives do one job: hold a thin film of warm air against the leg and move sweat away before it cools. The selection here leans towards women's legwear, with thermal walking leggings, high-waisted cuts, winter weights, athletic styles, a half-length legging and a walking skirt with leggings built in. Some are worn alone in mild weather, others under trousers when the temperature drops. Either way the fit against the skin decides whether the layer works.

What does a leg base layer actually do?

Two things, and warmth is only the second. The first is moving moisture off the skin, because damp fabric pulls heat away far faster than dry air ever does. That is why a cotton legging feels comfortable at the start of a walk and cold at the first stop, while a synthetic or wool-blend knit keeps working once you sweat. The second job is reducing friction: a smooth layer between skin and trouser fabric prevents the rubbing that turns into a sore patch after several miles. Under the roomier cuts held in hiking trousers, it sits flat instead of stretching across the knee.

Leggings or traditional longjohns?

The difference is where the waist sits and how much shape the knit holds. Traditional cuts are loose, thick and meant to disappear under work trousers. Leggings are elastic, close-fitting and made to be seen, so they can be worn on their own with a longer top. A high waist stays put when a rucksack hip belt presses on it, and that is the single detail most often noticed after a full day out. Under a heavier trouser, both work, though the slimmer knit slides more easily into a fitted leg.

What is worth checking on a pair?

• Flat seams: a raised seam on the inner thigh is the first thing to chafe once the fabric is damp.
• Waistband depth: a broad band spreads pressure and stays put; a thin one rolls down as you walk.
• Pocket: a side pocket sized for a phone keeps it off the hip bone and out of the way of a belt.
• Ankle finish: a snug cuff stops the leg riding up inside a boot, which is what leaves a cold gap at the sock.

How do you wear them under trousers?

Thin under thick, and never the reverse. The base layer goes on first, smooth and without folds, then the outer trouser carries the wind and the abrasion. The harder-wearing cotton cuts in military trousers take a thermal layer underneath without pulling tight when the ground is rough. A leg that feels tight standing in the bedroom will feel tighter on a stile.

When is a layer over the top needed?

As soon as rain arrives. Thermal knit soaks and stays soaked, and once wet it stops insulating entirely, so a shell pulled over the top is what keeps the system working; that is the point of waterproof overtrousers in a British forecast. Wash thermal legwear on a cool cycle without conditioner, and dry it flat rather than on a radiator, which cooks the elastic out of the waistband.