A sentence of the raw corpus that contains s words is included twice, once as a sequence of the ⌊s/2⌋ bigrams "w 1 −w 2 w 3 −w 4 w 5 −w 6 .
for a trigram model, this means that we interpolate p KN (w 3 |w 2 ) and p B (w 3 |w 1 w 2 ) on the first backoff level and p KN (w 3 ) and p B (w 3 |w 2 ) on the second backoff level, where p B is the (Brown) class model (see Section 4 for details on p B ).