If we want to include an unknown word <UNK>, it's just included as a regular vocabulary entry with count zero, and hence its probability will be a lambda-weighted uniform distribution λ ( ) V . The best performing version of Kneser-Ney smoothing is called modified Kneser-Ney smoothing, and is due to Chen and Goodman (1998). Rather than use a single modified Kneser-Ney fixed discount d, modified Kneser-Ney uses three different discounts d 1 , d 2 , and d 3+ for n-grams with counts of 1, 2 and three or more, respectively. See Chen and Goodman (1998, p. 19) or Heafield et al. 2013for the details.