The shoulders decide everything

If you only learn to judge one thing about a suit jacket, make it the shoulders. Nearly everything else — the waist, the sleeves, the length of the trousers — can be taken in, let out or shortened by a competent tailor for a modest amount of money. The shoulder is built into the structure of the jacket. Changing it means taking the jacket apart.

What right looks like

The shoulder seam should sit where your shoulder stops and your arm begins. Run a finger along the top of your shoulder to the bony point at the end; the seam belongs on top of that point, not before it and not past it.

Stand square in front of a mirror with your arms down. The line from your neck to the end of the shoulder should be smooth. No dent, no shelf, no rumple.

The two ways it goes wrong

• Too wide, and the shoulder pad hangs out past your arm. You get a small overhang and, underneath it, a dimple in the sleeve where the fabric has nothing to sit on. It reads as a jacket borrowed from someone larger.
• Too narrow, and the seam sits inside your shoulder point. The sleeve head gets dragged upward and you see diagonal creases running from the top of the arm down toward the chest. It reads as a jacket you have outgrown.

A jacket that is slightly big in the waist looks fine. A jacket that is wrong in the shoulders never does.

Why made-to-measure helps here

Off the rack, shoulder width comes bundled with chest size. If you have a 40-inch chest and narrow shoulders, you take what the 40 gives you. Cut to your own measurement, the shoulder is its own number, and the one measurement you cannot fix afterwards is the one taken from you rather than from a chart.

It is also why we ask you to measure shoulder to shoulder across the back, and why our fit team looks at your photos before anything is cut. Shoulder slope varies more between people than almost any other dimension, and a number on its own doesn't show it.