When a bar of almost any common material is stretched, it
not only gets longer, it also gets thinner. The ratio of the
decease in width per unit width, over the increase
in length per unit length, is called Poisson's ratio.
n = (Dw/Wo)/(Dl/Lo)
If the ratio is exactly 0.5, there is no volume change as
the material is stretched.
Poisson's ratio is listed for many metals and other materials.
It would be an interesting exercise to find the ratio for rubber,
by making careful measurements.