For example, we extract subtree "社 会(society):2-边缘(fringe):0" on the Chinese side and get its corresponding subtree "fringes(W 2):0of:1-society(W 1):2" on the English side, where W 1 means that the target word is aligned to the first word of the source subtree, and W 2 means that the target word is aligned to the second word of the source subtree.
we have a sub-tree pair: "社 会(society):2-边 缘(fringe):0" and "fringe(W 2):0-of:1-society(W 1):2".