University of Cincinnati, Blue Ash: Kenneth R. Koehler's "Logic and Set Theory"
Read these four sections of Koehler's lectures on logic and set theory. A contingency is simply a proposition that is caught between tautology (at the top) and contradiction (at the bottom). In other words, it is a proposition which is true for some values of its components and false for others. For example "if it rains today, it will snow tomorrow" is a contingency, because it can be true or false depending on the truth values of the two component propositions.