  Every knot can be embedded in the union of finitely many half planes with a
common boundary line in such a way that the portion of the knot in each half
plane is a properly embedded arc. The minimal number of such half planes is
called the arc index of the knot. We have identified all prime knots with arc
index up to 11. We also proved that the crossing number is an upperbound of arc
index for non-alternating knots. As a result the arc index is determined for
prime knots up to twelve crossings.
