A proposition constant (also called a logical constant) is a name for a proposition. In this book, we write proposition constants as strings of letters, digits, and underscores ("_"), where the first character is a lower case letter. For example, raining is a proposition constant, as are rAiNiNg, r32aining, and raining_or_snowing. Raining is not a logical constant because it begins with an upper case character. 324567 fails because it begins with a number. raining-or-snowing fails because it contains hyphens. A proposition constant is the simplest form of propositional sentence.