Create a poly plane with one division down the middle, use the split poly tool to make a triangle, delete the 2 side tris, rotate the tri 90 degrees so its facing up, move the pivot point to the bottom of the tri using the snap to vertex (press insert, hold V mmb hold and wiggle a little over the position of the bottom vert on the edges thats sepperating the half's of the tri) press insert to set the pivot.

Heres a pic