The phenomenon of static equilibrium for a system of point charges is well known in Newtonian theory, where the mutual gravitational and electrostatic forces can be balanced by fine-tuning the charge suitably with the particle masses. The corresponding generalisation, in the form of static solutions of the coupled, source-free Einstein-Maxwell equations, was discovered by Majumdar and Papapetrou independently in 1947. These gravitational fields assume no spatial symmetry and also contain geodesics which are incomplete. While work continued on understanding these solutions better, a renewed interest in this metric was generated by the important observation of Israel and Wilson in 1972 that static black-hole spacetimes with the mass being equal to the magnitude of the charge are of Majumdar–Papapetrou form. In the same year, it was shown by Hartle and Hawking that these spacetimes can be analytically extended to electrovacuum black hole spacetimes with a regular domain of outer communication. They interpreted this as a system of charged black holes in equilibrium under their gravitational and electrical forces. Each one of these many black holes or the multi-black holes system has a spherical topology and hence is a fairly regular object. In a more recent development, the uniqueness of the metric was discussed by Heusler, Chrusciel and others. These and other aspects of the Majumdar–Papapetrou metric have attracted considerable attention on the classical side, as well as in the work and applications from the perspective of string theory. In particular, the mass equal to charge aspect of these models was used extensively in certain string theoretic considerations connected to black hole entropy and related issues.