The first analysis of the thorium III spectrum led DeBruin and Klinkenberg (40.1) to the discovery of two systems of transitions (5f6d+5f7s) - 5f7p and 6d2+6d7s - 6d7p+7s7p. With the help of extended Zeeman effect observations, the same authors and Schuurmans (41.1) added new levels and the configuration 5f2. A comprehensive description of Th III, including a tentative connection of both level systems was given by Klinkenberg (50.3) and followed by a parametric study of the mixed configurations in both parities by Racah (50.4). The first infrared observations by Litzén (74.11) led to revise the connection: the new value (63.2 cm-1) of the energy of 6d2 3F2 relative to the ground state 5f6d 3H4 is the smallest distance between lowest levels of both parities ever found in an atomic spectrum. From the study of the sliding spark spectrum in the vacuum ultraviolet region, Wyart and Kaufman (81.2) determined new configurations (6d8s, 6d7d, 5f6f, 5f8p, 5f8s, 5f7d and 6d6f) from which a semi-empirical value of the ionization limit was derived. The hollow cathod emission spectrum of thorium was measured in the range 277,7 - 1350 nm (83.9) and about 150 Th III lines led to improve the accuracy of the low level energies (83.5). The estimated error on the quoted values with 3 decimal figures is 0.002 cm-1. The parametric study of the low-lying configurations which had been done first (50.4) in separate groupe has been unified in two odd and even groups of mixed configurations, especially for inclusion here (unp.9) and takes into account the revised levels 5f6d 1P1 and 5f2 1S0 (81.2) and 5f2 lI6 (83.5). In previous theoretical studies, (50.4; 71.5) the energy parameters were spoiled by the earlier erroneous values of these levels. The level 5f2 lI6 has been confirmed by Zeeman effect and the parametric interpretation of the five lowest even configurations, using two different sets of electrostatic operators leads to results (86.7) similar to those reported here.