Other mathematicians also contributed to make the law better. Some of them were Chebyshev (who proved a more general version of the law for averages), Markov, Borel, Cantelli and Kolmogorov. After these studies, there are now two different forms of the law: One is called the "weak" law, and the other the "strong" law. These forms do not describe different laws. They have different ways to describe the convergence of the observed or measured probability to the actual probability. The strong form of the law implies the weak one.