What merino, cashmere and cotton actually do in an office

Buy the fine-gauge merino crew first. If you work in an office with a thermostat somebody else controls, a 12-gauge merino jumper in navy or charcoal will do more work than anything else in the drawer. It goes under a jacket without bulking the shoulders, over a shirt without looking like a costume, and on its own with trousers when the meeting is internal.

Everything below is the reasoning, plus the parts the label leaves out.

Read the fiber, then read the gauge

Two numbers decide how a jumper behaves, and neither is the brand.

The first is fiber diameter, measured in microns. Merino used for knitwear usually sits around 17 to 19 microns. Cashmere runs finer, roughly 14 to 16. Finer fiber means a softer hand against the neck, and it's why cashmere feels like an upgrade the second you touch it. Finer also means weaker per fiber, which is the trade nobody prints on the swing tag.

The second is gauge, the number of needles per inch on the knitting machine. A 12-gauge knit is thin and smooth. A 7-gauge is a proper mid-weight. A 3-gauge is the chunky fisherman thing that won't close under tailoring, ever. If you wear a jacket to work, 12-gauge is your default and 7-gauge is your Friday.

Ply is the third thing worth checking and the one most people get wrong. Two-ply means two strands twisted together before knitting. It's denser, holds shape better and pills less than a single-ply of the same fiber. A two-ply merino will outlast a thin single-ply cashmere by years, and it will look better in month fourteen, which is when jumpers actually get judged.

Merino: the one that does the most days

Merino's advantage is that it recovers. The fiber has natural crimp, so the elbows and cuffs spring back overnight instead of staying where you left them. It handles a temperature swing better than anything else, which matters when your desk sits at 21°C and the meeting room on the north side is closer to 17.

It also doesn't smell after a day, so you can wear it twice or three times between washes.

What goes wrong: thin merino pills where things rub. The left shoulder if you carry a bag on the left. The inside of the forearm against a desk edge. The seatbelt line. Pilling isn't a defect in the first few wears, it's short fibres working their way out, but a very light single-ply will keep doing it for the life of the garment.

Moths eat it. They're eating the protein, not the jumper, and they prefer it dirty.

Buy merino in the colors you actually wear with your suits and your dark jeans. Navy, charcoal, a mid gray. If you only own one, make it the crew neck, because a V-neck commits you to a shirt and tie underneath and a crew doesn't commit you to anything.

Cashmere: warmth per gram, at a cost

Cashmere gives you more warmth for less thickness. That's the entire case, and it's a real one. A fine cashmere crew will keep you comfortable at a weight where merino would feel insufficient, which is why it works under a slim jacket with no room to spare.

The soft, dense, almost creamy feel is genuine. So is the fact that a cheap cashmere jumper is usually a bad buy.

The grading language you see online is largely marketing. "Grade A cashmere" isn't a regulated term. What actually determines whether a cashmere jumper survives is fiber length and construction: long fibres tightly spun, ideally two-ply, with a firm rib at the cuff and hem. Cheap cashmere uses short fibres, sometimes recycled, spun loosely to feel soft on the shop floor. It feels wonderful for a fortnight and then goes fuzzy and thin at the elbows.

Hold it up to a window. If you can see the light through the body of a jumper that's meant to be a winter layer, it's thin. Stretch the cuff and let go. It should snap back, not sit there.

All cashmere pills at first. That's normal, and a cheap wool comb fixes it in about four minutes per jumper. If it's still shedding pills at month six, you bought badly.

Cotton: the one for the warm office

Cotton knitwear gets dismissed and shouldn't be. If your building runs hot, if you sweat easily, or if it's May and a wool jumper feels absurd, cotton is the answer rather than a compromise.

It washes in the machine. Moths ignore it. It takes brighter colors cleanly, which is why a cotton crew in something other than navy usually looks better than the wool equivalent in the same shade.

The problem is elasticity. Cotton has almost none. It stretches and it stays stretched, so the cuffs go first, then the neck opening, then the whole thing hangs about an inch longer than it did. Look for long-staple cotton, sometimes labelled pima or Supima, and for a tight rib at the cuff. Mercerised cotton has a slight sheen and resists pilling better, though it reads a bit more formal.

The other catch: cotton holds water. Get caught in rain and a cotton jumper stays cold and heavy for hours. Wool doesn't.

Blends, and when they earn their place

A small amount of nylon, five to ten percent, in a merino jumper adds meaningful abrasion resistance. If the jumper is going in a rucksack every day, that's worth having. Silk and cashmere blends drape beautifully and are cooler than pure cashmere, good for a transitional month.

Be suspicious of a jumper that's mostly acrylic with a token wool percentage. It'll be warm and it'll pill fast, and it won't breathe, so you'll overheat in the afternoon and then feel clammy on the walk to the station.

Fit, for the specific case of wearing it to work

The shoulder seam sits on your shoulder bone, not down the arm. The sleeve ends at the wrist bone, so about an inch of shirt cuff shows if you're wearing one. The body ends below your belt but above the fly, roughly at the top of your trouser pocket. Too long and it bunches when you sit; too short and it rides up every time you reach for anything.

Under a jacket, check you can do the jacket button up and still move your arm. Do that in the shop. It's the single test most people skip and the reason half the nice jumpers in the country only get worn at weekends.

Make it last

• Wash at home. Cool wool cycle or hand wash, wool detergent, no fabric softener. Dry cleaning solvents are harsher on cashmere than a careful hand wash.
• Never hang a jumper. Fold it. Hangers put the whole weight on two shoulder points and they stay stretched.
• Rest it two days between wears so the fibres recover.
• Comb the pills rather than pulling them, which takes fiber with them.
• Wash before storing in spring. Moths go for the sweat and the food traces, not the clean wool. A sealed bag in the loft beats cedar blocks.

The jumper that looks expensive in year three isn't the one that cost the most. It's the one somebody bothered to fold.