  We show that the probability of appearance of synchronisation in chaotic
coupled map lattices is related to the distribution of the maximum of a certain
observable evaluated along almost all orbit. We show that such distribution
belongs to the family of extreme value laws, whose parameters, namely the
extremal index, allow us to get a detailed description of the probability of
synchronisation. Theoretical results are supported by robust numerical
computations that allow to go beyond the theoretical framework provided and are
potentially applicable to physically relevant systems.
