The bound state of the electron and its antimatter counterpart, the positron. Positronium (abbreviated Ps) is perhaps the simplest bound state, since the constituents are structureless and the electromagnetic binding force is well understood. As in the hydrogen atom, the states of positronium are quantized. The spins of the two spin-¹/₂ constituents combine to yield total spin zero (singlet state) or one (triplet state). Singlet and triplet states are called parapositronium (p-Ps) and orthopositronium (o-Ps), respectively. States of positronium are labeled by principal quantum number n, orbital angular momentum, total spin, and total angular momentum (orbital plus spin). The energy levels are approximately −6.80 eV/n², with numerous small corrections due to relativity, virtual annihilation, and other electromagnetic effects. Additional corrections from strong and weak interactions are negligible.