If we add a mountain fold at this vertex, there are three valleys and two mountains.

如果我们在这个顶点加上一个山折，就是三个山谷和两个山峰。

Unfold that sheet, and there's a pattern of lines, each of which represents a concave valley fold or a convex mountain fold.

展开那张纸，有一种线条图案，每条线代表一个凹谷折或凸山折。

Second, the number of mountain and valley folds at any interior vertex must differ by exactly two— like the three valley folds and one mountain fold that meet here.

其次，任何内部顶点处的山折和谷折的数量必须恰好相差二——就像在这里相交的三个谷折和一个山折一样。