The most accurate computations on the ground state of the helium atom and its isoelectronic series followed from the work of Pekeris [1]. For an S-state, the wavefunction depends on just three coordinates, say , and , which can be represented as the sides of a planar triangle. The perimetric coordinates , , have the advantage that they automatically satisfy the triangle inequalities and each independently varies from 0 to ∞. Pekeris's original computation made use of an expansion in perimetric coordinates containing 1058 terms, leading to the essentially exact nonrelativistic ground-state energy hartrees. In this Demonstration, we introduce the use of perimetric coordinates in computations on the two-electron isoelectronic series , , , …, , corresponding to in the Hamiltonian