The conformations of the archetypal acyclic phosphorus ligands PEt3 and P(OMe)(3) are classified on the basis of the observation that torsions about the P-C (or P-O) bonds show favoured conformations lying close to gauche (+/- 60 degrees) or anti values (180 degrees). Analysis of the symmetry of the conformation space defined by the three M-P-X-C (X = CII2 or O) torsion angles (t(1-3)) implies the existence of seven unique conformer types (A (aaa), B (g(+)g(+)g(+)), C (ag(+)g(+)), D (aag(+)), E (g(-)ag(+)), F (ag(-)g(+)), G (g(-)g(+)g(+)) and their symmetry equivalents) arising from the combinations of g(-), g(+) and a torsions. These conformers are observed in 1972 M-PEt3 and 735 M-P(OMe)(3) fragments from crystal structures of metal complexes in the CSD following the popularity sequence: F > C >> D > B > G > E >> A for M-PEt3; and: C > D > F >> E >> A, B, G. for M-P(OMe)(3). Pathways for low-energy interconversion of these conformers, dominated by single chain flip routes, are readily inferred for M-P(OMe)(3). The conformers of M-PEt3 are apparently less readily interconverted. The popularity of conformations is only loosely related to the energies of these conformations as calculated by DFT or MM methods for two-, four-(square planar) and six-coordinate metal complexes of these ligands (and free PEt3 and P(OMe)(3)). It would appear that the conformational preferences observed are determined by a balance between intra-ligand effects (repulsion between chains and avoidance of syn-pentane-like); inter-ligand effects (repulsions between gauche substituents at P and cis co-ligands notably when the coordination number at the metal is high); and residual anomeric effects (weakly favouring anti conformations in P(OMe)(3) species).