The checks that tell you a garment actually fits

Put your finger on the shoulder seam. If it sits on the bony corner at the top of your arm — the point where the flat of your shoulder turns and starts heading down — the garment is worth the rest of your attention. If it's sitting an inch out towards your bicep, or dug in towards your neck, put it back. Everything else on a jacket or a structured shirt hangs off that seam, and it's the one thing a tailor either can't fix or will charge you more to fix than the garment is worth.

That's the whole logic of fitting yourself. Start with the parts that can't be changed, work down to the parts that can, and don't let a nice colour talk you out of the order.

What can't be altered, in order

Shoulders. Already covered. On a jacket, it's a rebuild. On a shirt it's usually not offered at all.

Armhole height. Cheap garments have low, wide armholes because it's easier to cut and it fits more people badly than well. Test it: with the shirt or jacket on, reach straight forward as if handing someone a mug. If the whole garment lifts off your back and the collar climbs up your neck, the armhole is too low. You'll feel it every day and no alteration touches it.

Chest and back width on a jacket. Small changes are possible. A jacket that's two sizes too big across the back isn't.

Rise on trousers — the distance from crotch seam to waistband. Too short and it drags every time you sit. Too long and it pools. A tailor can't move it meaningfully.

Everything after that is negotiable. Sleeve length, jacket length within limits, waist suppression, trouser waist, taper, hem. Those are ordinary jobs that any competent alterations shop does all week.

The shirt, in six checks

Button it all the way, including the collar, even if you'll never wear it that way.

• Collar. Two fingers should slide in flat between collar and neck. One finger and you'll hate it by 2pm. Four and it gapes under a tie or a jumper.
• Shoulder seam. As above.
• Chest. With the top button done and arms down, you want the placket to lie flat. Any horizontal pull between the buttons means it's too tight, and that pull gets worse the moment you sit.
• Body. Grab the excess fabric at your ribs with one hand. A palm's worth of slack each side is roughly a normal fit. A fistful each side is a tent.
• Cuff. Buttoned, the cuff should not slide over the bone of your wrist when your arm hangs down. If it does, the sleeve is long. That's a cheap fix.
• Hem. A shirt meant to be tucked should cover your backside and stay put when you raise both arms. A shirt meant to be worn out should finish somewhere around mid-fly. Most untucked shirts are too long, and shortening one is easy money well spent.

The jacket, in five

Do these buttoned, standing straight, arms down.

The collar gap. Look at the back in a second mirror or a photo. The jacket collar should sit flush against your shirt collar with no gap and no bunching. A gap means the jacket is hanging off your shoulders wrong, and that's a real alteration, not a quick one.

The X. Look at the fastened button. A slight ripple is fine. Hard creases radiating outward in an X shape mean it's pulling and you need the next size up, or a different cut.

Sleeve pitch. Let your arms hang naturally. If the sleeve twists or shows diagonal creases on the outside of the upper arm, the sleeve is set at an angle your arm doesn't use. Some tailors can re-pitch it. Most won't bother, and you shouldn't pay for it on a mid-priced jacket.

Sleeve length. Jacket sleeve ends where your wrist bone starts, leaving roughly a centimetre of shirt cuff showing. If you never wear a cuffed shirt underneath, just end it at the wrist bone.

Length. Standing straight with arms down, your knuckles should be roughly level with the hem. A jacket that covers your seat looks balanced. One that stops above it makes your legs look short, however trendy that season claims to be.

Trousers, and where most people get it wrong

Try them on without a belt. If they won't stay up, the waist is too big — the belt is hiding the problem, not solving it, and the excess fabric will bunch around the buckle for the life of the garment.

Check the seat from the side in a photo. Horizontal pull lines under the backside means too tight. A sagging hammock means too loose. You want the fabric skimming.

Then sit down. Properly, in a chair, for ten seconds. Trousers that fit standing and cut into you sitting are trousers you'll take off the moment you get home.

Hem last, and hem for the shoes you'll actually wear them with. A slight break — one soft fold where the fabric meets the shoe — is forgiving and looks intentional. No break looks sharper and is less forgiving of your leg length. Pooling fabric round the ankle looks like nothing at all went into the decision.

The movement test nobody does

Before you buy anything, do four things in the fitting room: reach forward, raise both arms overhead, sit down, and cross your arms as if you're cold. Four seconds each. A garment that fails any of them will annoy you for years, and you'll never work out why you don't reach for it.

Then take a photo. Front, side, and back if you can manage it. Mirrors flatter, and you stand differently when you're looking at yourself. A photo shows you the collar gap, the twisted sleeve, the hem that's sitting wrong. It's the single most useful habit in this whole business.

Size labels are marketing, not measurement

Two garments both marked medium, from the same shop, in the same season, will often fit differently. That's normal. Take three sizes into the room every time and treat the label as a starting guess.

And if the only thing standing between a garment and a good fit is one alteration, get the alteration. Sleeve shortens and trouser hems are the cheapest work a tailor does. Taking in a shirt's side seams or a jacket's waist costs more but transforms the thing. Ask for the price before you hand it over, and ask whether they think it's worth doing — a good alterations tailor will tell you honestly when it isn't.

None of this is about having a particular body. Tall, short, broad, narrow, heavier, leaner — the checks are identical, and the garment is the thing that's wrong, not you. Clothes are made to an average nobody actually is. The tailor exists precisely because of that.

Walk into your wardrobe tonight and find the shirt you always reach for. Then find the one you never do. Put them side by side, look at the shoulder seams, and you'll have your answer in about four seconds.