The discounting we have been discussing so far can help solve the problem of zero frequency n-grams. But there is an additional source of knowledge we can draw on. If we are trying to compute P(w n |w n−2 w n−1 ) but we have no examples of a particular trigram w n−2 w n−1 w n , we can instead estimate its probability by using the bigram probability P(w n |w n−1 ). Similarly, if we don't have counts to compute P(w n |w n−1 ), we can look to the unigram P(w n ).