In philately, tête-bêche (French for “head-to-tail”, lit. “head-to-head”) is a joined pair of stamps in which one is upside-down in relation to the other, produced intentionally or accidentally. Like any pair of stamps, a pair of tête-bêches can be a vertical or a horizontal pair. In the case of a pair of triangular stamps, they cannot help but be linked “head-to-tail”.
* Text from Wikipedia