1. [Becker 1930] Becker, O.: “Zur Logik der Modalitäten”, Jahrbuch für Philosophie und Phänomenologische Forschung, Vol. 11, 1930, pp. 497–548.
2. [Becker 1930–1952]Untersuchungen über den Modalkalkül, Westkulturverlag Anton Hain, Meisenheim am Glan, 1952.
3. [Churchman 1938] Churchman, C. West; “On Finite and Infinite Modal Systems”,Journal of Symbolic Logic, V. 5 (1938) pp. 77–82.
4. Churchman 1942 “Towards a General Logic of Propositions” inPhilosophical Essays in Honour of Edgar Arthur Singer, University of Pennsylvania Press, 1942, pp. 46–68.
5. [Goldblatt 1972] Goldblatt, R.I.: “Temporal Betweenness” unpublished manuscript, 1972.