Let's get straight to it — here's what actually matters on this one.

• Shoulder seams should sit exactly at the edge of your shoulder bone — not drooping past it, not pulling inward.
• Fasten the top button and check you can comfortably slide a flat hand between the lapel and your chest — any tighter restricts movement and breathing.
• Sleeve length should stop right at your wrist bone, showing about half an inch of shirt cuff.
• A blazer that pulls across the back in a horizontal 'X' pattern when buttoned is too tight through the shoulders or chest, full stop.

The fit-and-fabric-health takeaway: A too-tight blazer in wool or poly-blend traps body heat with almost no airflow — one reason blazers feel unbearable within an hour if sized wrong.

Next up: check out our guide on how to buy shirts for broad shoulders.