Pairs in P2₁: probability distributions which lead to estimates of the two-phase structure seminvariants in the vicinity of 0 or π

The first sequence of nested neighborhoods of the two-phase structure seminvariant φ₁₂ = φ_h1k1 -- φ_h2k2 in the space group P2₁ is defined, and conditional probability distributions associated with the first four neighborhoods derived. In the favorable case that the variance of a distribution happens to be small, the distribution yields a particularly reliable value for φ₁₂. The most reliable estimates are obtained when φ₁₂ ≃ 0 or π.