A watch strap typically has two different sizes: the lug size and the buckle size. The lug size refers to the width of the strap at the point where it attaches to the watch case, and is measured in millimeters. The buckle size, on the other hand, refers to the width of the strap at the point where it attaches to the buckle, and is also measured in millimeters. The 20 mm x 16mm indicates that the lug size is 20mm and the buckle size is 16mm. These two measurements are important because they ensure that the strap fits properly on the watch and that the buckle is the correct size to securely fasten the strap.