We study the atomic structure of $\{112\} \Sigma = 3$ twin boundaries consisting of a
stacking of six $\{111\}$ planes and characterized by a resulting Burger vector equal
to zero. The twins were imaged by HRTEM. We develop image processing to localize the
silicon and carbon atoms in each grain and thus to deduce their stacking and to
calculate the possible rotation and translation of one grain with respect to the other
with an accuracy greater than 0.1%. We show that there is no rigid body translation
between the two grains. However, models derived from Möller or Pond et al.
are not suitable. We propose an alternative model which is in close agreement with the
asymmetrical contrast of the twin boundary.