The skeleton B is considered being filled by the synchronous tree T , and P (T |B) is decomposed into conditional probabilities of bilingual constituents α and contexts β conditioning on B ij,pq , a Boolean variable indicating whether the under-consideration bi-span <i, j><p, q> is a syn span or not.
α ij,pq denotes the bilingual constituent spanning from i to j on source side sentence, and spanning from p to q on target side sentence.