For convenience, geologists use the notation R (ridge), T (trench) and F (fault) for cataloging triple junctions. Thus a triple junction called RRR could exist at which all three plates are moving apart. There are several on Earth today. Likewise, a triple junction called TTT could exist with all three plates pushing together, if they're lined up just right. One of these is under Japan. An all-transform triple junction (FFF), though, is physically impossible. An RTF triple junction is possible if the plates are lined up correctly. But most triple junctions combine two trenches or two faultsthat is, RFF, TFF, TTF, RTT. Triple junctions with two ridges (RRT, RRF) cannot exist for more than an instant, splitting into two RTT or RFF triple junctionsthat is, they are unstable. That makes ten possible combinations of R, T and F, of which seven match existing types of triple junction and three are unstable.