Under the N-gram Markov assumption, the probability of the next word is approximated using only the preceding $N-1$ words: $P(w_n|w_{1:n-1}) \approx P(w_n|w_{n-N+1:n-1})$ Applying this approximation to the chain-rule factorization gives the sequence probability: $P(w_{1:n}) \approx \prod_{k=1}^{n}P(w_k|w_{k-N+1:k-1})$