There are, in addition, two other important differences in our approach from that in Kneser and Ney (1995), which would remain as differences even if our target expected frequency were the unsmoothed relative frequency P instead of the smoothed estimate P. First, the sum in the numerator is over histories of length n, the highest order in the n-gram model, whereas in the Kneser-Ney approach the sum is over histories that immediately back off to h , i.e., from the next highest order in the n-gram model.
the unigram distribution is with respect to the bigram model, the bigram model is with respect to the trigram model, and so forth.