Here is one pair of twin brothers that few will have any difficulty telling apart.
For although Byron and Lincoln Ryman were born within a minute of each other, firstborn Byron - at 3lb 6oz a small baby himself - was precisely three times bigger than his sibling.
Eleven weeks premature, both boys were given a slim chance of survival - particularly Lincoln, weighing only 1lb 2oz at birth.
The pair, now just over five weeks old are doing well, thanks to the dedicated staff at the Royal Hospital for Women in Sydney.
Lincoln has gained 10oz, while Byron has added 2lb 8oz.