The ring spanner
usually has a completely enclosed head,
and may have six or 12 flats. A 12 flat
spanner engages upon the corners of
the nut and can engage both hexagon
and square bolts. A six flat spanner
is normally shaped to fit against all
6 sides of hexagon nuts, this ensures
a very tight fit and can allow considerable
force to be applied.
spanners are stronger than the open ended
type, but they need access to fit them
over the nut.