  Twinning is an important mode of plastic deformation in metallic nanopillars.
When twinning occurs on multiple systems, it is possible that twins belonging
to different twin systems interact and forms a complex twin-twin junctions.
Revealing the atomistic mechanisms of how twin-twin interactions lead to
different twin junctions is crucial for our understanding of mechanical
behaviour of materials. In this paper, we report the atomistic mechanisms
responsible for the formation of two different twin-twin interactions/junctions
in Cu nanopillars using atomistic simulations. One junction contains two twin
boundaries along with one $\Sigma$9 boundary, while the other contains five
twin boundaries (five-fold twin). These junctions were observed during the
tensile deformation of [100] and $[1\bar1 0]$ Cu nanopillars, respectively.
