In the regular MSD model, the previous orientation for a phrase pair is monotonous if the previous word in the source phrase is aligned with the previous word in the target phrase and not aligned with the next word.
the probability of a phrase pair to have a monotonous orientation P c (M ) is given by the probability of the previous word in the source phrase being aligned with the previous word in the target phrase W n−1 i−1 , and the probability of the previous word in the source to not be aligned with the next word in the target (1 − W n−1 j+1 ).