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David,
Lay the jacket flat and button it, then measure it in inches from armpit to armpit. Then subtract 4" for ease and you will have the chest size of the suit. So if the jacket measures 44" , its a size 40. Then measure the back length of the jacket, from the neck seam at the base of the collar at the center back to the hem for the Length ( Short, Reg , Long). This is variable depending on the chest size, but there are charts online .
For the pants - fasten the waistband and measure - this is the waist measure. Measure the inseam from the crotch to the pants hem. So a SZ 40 suit usually has a 34" waist for the pants, while the length was set to the wearer.
So, for example, a suit could be a 40L with a 34/33 trousers. Or the waist could have been altered to be either larger or smaller than standard.
In men's suiting size is a huge factor in value.
May I ask your basis for 1970s? I ask because the very long line of the widish lapel with a 4 button DB front with a single button closure was a very 1990s line. 1970s DB jackets were much more likely to have a 6 button front with 2 actual closures and be more fitted in the torso. Are there pleats on the trousers?