As visible in Fig. , when the reference electron is in the region dominated by ψ 100 (r = 0.1 bohr), the probability density to find the remaining electron essentially mirrors the shape of the ψ 211 function (left panel), viceversa when the reference electron is in the region dominated by ψ 211 , the conditional probability amplitude resembles the ψ 100 function (right). For intermediate distances, the conditional amplitude is a mixture of the two functions (center). In other words, the behaviour of the (modulus squared of the) conditional amplitude reflects the common understanding that the remaining electron "occupies" one or the other orbital according to where the reference electron is placed and clearly exemplifies the so-called Fermi correlation. This is the type of correlation that stems from the indistinguishability of electrons and it manifests itself in the spatial part of the wavefunction for systems with parallel spins .