Why are there solid lines drawn connecting the solid circles after twinning has taken place? Ideally, that explains a real plane, isn't it? But why are we going to get a plane out of nothing if we mechanically twin or a twinning is followed after heat treatment(annealing twins)?
solid circles represent the positions of atoms in the twinned region; these black circles represents the only atoms that are present there, whereas dashed circles are their original positions that after twinning are no more occupied on account of displacement.
These black atoms are displaced from their original positions on account of twinning and hence form novel planes that are not found in the untwinned regions.