This standard reference relates to the width of the garments, not the body. The measurement is obtained by laying the garment flat on a desk or table. The garment is then measured across the front, side seam to side seam, from a point that's just a few centimetres below where the sleeve joins the body of the garment. A garment with a half or across-chest measurement of 50cm would be skin-tight on a person with a 100cm chest. Therefore, the half or across-chest measurement - when doubled - should be a few centimetres more than the person's actual chest measurement (depending on the desired fit).