Each type of plate boundary has rules about relative velocities:
ridge - relative velocity must be divergent and is usually perpendicular to the ridge
transform fault - relative velocity must be parallel to the fault
trench - relative velocity must be convergent but no direction is preferred
All triple junctions must satisfy a velocity condition such that the vector sum around the plate circuit is zero.
VBA + VCB + VAC = 0
In the real world we usually can map the geometry of the spreading ridges, transform faults and trenches but cannot always measure the relative velocities. The triple junction closure equation (1) can be used to solve for spreading velocities given the triple junction geometry, the rules, and at least one relative plate velocity.
Example: Galapagos Triple Junction - RRR
A triple junction is the intersection of three plate boundaries. The most common types of triple junctions are ridge-ridge-ridge (R-R-R), ridge-fault-fault (R-F-F), and ridge- trench-trench (R-T-T).