boolean, symbol, t
The type boolean contains the symbols t and nil, which represent true and false, respectively.
t (constant variable), nil (constant variable), if, not, complement
Conditional operations, such as if, permit the use of generalized booleans, not just booleans; any non-nil value, not just t, counts as true for a generalized boolean. However, as a matter of convention, the symbol t is considered the canonical value to use even for a generalized boolean when no better choice presents itself.