A system of Law Encoding Diagrams (LEDs) for probability theory is described. A LED is a representation that encodes the underlying relations of a law, or a system of simultaneous laws, in the structure of a diagram by the means of geometric, topological and spatial constraints, sauch that each instantiation of a diagram represents one instance of the phenomenon or one case of the laws. The class of LEDs for probability theory have been named PT diagrams. Conditional probability, Bayes's theorem, and likelihood ratios are examined using PT diagrams to demonstrate the properties of this representational system, with some consideration of the forms of reasoning that can be conducted with the diagrams in contrast to inferences based upon the conventional algebraic approach.