 Zero-place predicate letters are sometimes called “sentence letters”. They correspond to free-standing sentences whose internal structure does not matter. One-place predicate letters, called “monadic predicate letters”, correspond to linguistic items denoting properties, like “being a man”, “being red”, or “being a prime number”. Two-place predicate letters, called “binary predicate letters”, correspond to linguistic items denoting binary relations, like “is a parent of” or “is greater than”. Three-place predicate letters correspond to three-place relations, like “lies on a straight line between”. And so on. 