  We present the results of novel, time-resolved, in situ HRTEM observations,
molecular dynamics (MD) simulations, and disconnection theory that elucidate
the mechanism by which the motion of grain boundaries (GBs) in polycrystalline
materials are coupled through disconnection motion/reactions at/adjacent to GB
triple junctions (TJs). We focus on TJs composed of a pair of coherent twin
boundaries (CTBs) and a {\Sigma}9 GB. As for all GBs, disconnection theory
implies that multiple modes/local mechanisms for CTB migration are possible and
that the mode selection is affected by the nature of the driving force for
migration. While we observe (HRTEM and MD) CTB migration through the motion of
pure steps driven by chemical potential jump, other experimental observations
(and our simulations) show that stress-driven CTB migration occurs through the
motion of disconnections with a non-zero Burgers vector; these are pure-step
and twinning-partial CTB migration mechanisms. Our experimental observations
and simulations demonstrate that the motion of a GB drags its delimiting TJ and
may force the motion of the other GBs meeting at the TJ. Our experiments and
simulations focus on two types of TJs composed of a pair of CTBs and a
{\Sigma}9 GB; a 107{\deg} TJ readily migrates while a 70{\deg} TJ is immobile
(experiment, simulation) in agreement with our disconnection theory even though
the intrinsic mobilities of the constituent GBs do not depend on TJ-type. We
also demonstrate that disconnections may be formed at TJs (chemical potential
jump/stress driven) and at GB/free surface junctions (stress-driven).
