The AEP for a source $X_1, X_2, \ldots$ with constant $H$ is equivalent to the following: for every $\varepsilon > 0$ there exists $n_0(\varepsilon)$ such that for all $n \geq n_0(\varepsilon)$ there exists a **typical set** $T_n \subseteq A^n$ satisfying: 1. $\mathbb{P}((X_1, \ldots, X_n) \in T_n) > 1 - \varepsilon$; 2. $2^{-n(H+\varepsilon)} \leq p(x_1, \ldots, x_n) \leq 2^{-n(H-\varepsilon)}$ for all $(x_1, \ldots, x_n) \in T_n$.