Supplemental Sets were released
occasionally over the years (especially the early years) to provide
additional parts for people who didn't want to buy new models
with instructions. These tended to be small and have a variety of
spare parts which were also available in regular sets. In some
cases, the supplemental sets had unique parts which couldn't be obtained
any other way. Technic figures were released three times this
way. It was also very common for regular Technic sets to have the
option to be motorized, with the motor only available as a supplemental
set.
