Paul Newmann in 'Harper'

The fit of a suit is important for one essential reason – it is made of flimsy cloth. The smarter clothes are the finer they tend to be, and the worsted wool that your suit is likely made of will be finer than anything else you wear. Unless you have a propensity for silk shirts.

This fineness means that any poor aspect of fit shows up immediately in tell-tale folds and wrinkles. If the shoulders are too big, they collapse at the ends and throw ripples of buckled cloth down over your shoulder blades. If the shoulders are too small, the top of the sleeve will strain against your deltoid muscle, that tension revealed in tight lines across the cloth at the sleeve. You are not wearing jeans or a baseball sweater now: fit is precise and unforgiving.

Most men don’t know how a suit should fit. And once they know a suit’s faults, they don’t have them altered. Do both, and you will have a suit that fits better than those of 90% of men out there. (We are dealing with ready to wear here, rather than made to measure or bespoke. More of that another time.)

So how do you tell if a suit fits? Well, don’t worry about the trousers. Chances are you’ll have to get the waist and length altered anyway – both only deal in measurements of two inches, and you are highly unlikely to be an exact, two-inch fit.

To the jacket. Note, first, the collar and shoulders. These are the most important parts of the fit to get right because they are the hardest parts to change. Altering the width of a shoulder or the position of the collar is a serious operation than can fundamentally alter the balance of the jacket.

The shoulder width should be such that the sleeve falls in a straight, uninterrupted line down your arm, just brushing your deltoid. If you are unsure, err on the larger of two sizes. The collar should sit tight against your neck. Not so tight that cloth folds up just below it on your back, but it should not stand away either. Nothing betrays a ready-to-wear suit more. Unfortunately, changing sizes is not likely to fix this problem. You’ll have to try different brands, most of which will work off slightly different blocks that will sit on your neck differently.

Now to the two points you are most likely to notice: the waist and the sleeve length. The first should be snug without any tell-tale folds that indicate it is straining against your stomach. The second should be such that a quarter to a half inch of shirt cuff is on display. It helps if all your shirts have the same length sleeves.

It is likely that you will end up having both of these elements altered. As you and others will notice them, they are worth having exactly right. But they shouldn’t be ignored when trying on a suit – neither can be altered dramatically without throwing off the balance of the jacket or making the position of the buttonholes look ridiculous. Make sure they are close.

That’s it, other than watching the chest to make sure there’s not too much room there and ensuring the jacket is long enough to cover your bottom. Buy it; have it altered; and for God’s sake keep it buttoned when you wear it.

Author: Simon Crompton, menswear journalist with a passion for style