A medium from 1989 and a medium from last year are different garments. Sizing has drifted for decades, and second-hand pieces have often shrunk or stretched on top of that. That is why every Rewear listing gives measurements instead of trusting the label.
Pit to pit
Lay a top or jacket you already love flat on a bed and do up the buttons or zip. Measure straight across the chest from one armpit seam to the other. That number is pit to pit. Double it for the full chest.
Length
Measure from the highest point of the shoulder, next to the collar, straight down to the hem. For cropped jackets this is the number that matters most.
If the listing is within 2 cm of a piece you own, it will fit the same way.
Jeans and shorts
For denim we list the waist measured flat across the top of the waistband, and the inseam from the crotch seam to the hem. Old denim has no stretch, so do not round down.
Keep the numbers of your three favourite pieces in your phone notes. Friday drops go quickly and it helps to know your size before you look.