In
American Sign Language
the letters and numbers "ABCOS15"
are the typical handshapes formed by your non-dominant hand when it is
serving as the base hand for a two-handed sign.
For two-handed
signs in which the hands do not have the same handshape you will notice
that typically the non-dominant hand is in one of these
shapes: "ABCOS15."
You see those shapes on the non-dominant signing hand when it is providing a
stationary base for your "in-motion" dominant hand.
For example, if you are right handed and are
doing a two-handed sign in which only one hand moves then the left hand will
almost always be one of those handshapes (ABCOS15) and vice versa for
left-handed people.
Notes:
Memory aid: "It took me 15 years to get my BA in SOC."