In his “Time and Modality”, based on his own philosophical motivations, Arthur Norman Prior proposed the modal logic Q as a correct modal logic in 1957. Prior developed Q in order to offer a logic for contingent beings, in which one could rationally state that some beings are contingent and some are necessary. One may say that Q is an actualist modal logic with a natural semantics. This review article is a developed description/discussion of/on “The System Q” that is the fifth chapter of “Time and Modality”. I have attempted to analyse the logical structure of system Q in order to provide a more understandable description as well as logical analysis for today’s logicians, philosophers, and information-computer scientists. In the paper, the Polish notations are translated into modern notations in order to be more comprehensible and to support the developed formal descriptions and semantic analysis.