  We define a knot to be half ribbon if it is the cross-section of a ribbon
2-knot, and observe that ribbon implies half ribbon implies slice. We introduce
the half ribbon genus of a knot K, the minimum genus of a ribbon knotted
surface of which K is a cross-section. We compute this genus for all prime
knots up to 12 crossings, and many 13-crossing knots. The same approach yields
new computations of the doubly slice genus. We also introduce the half fusion
number of a knot K, that measures the complexity of ribbon 2-knots of which K
is a cross-section. We show that it is bounded from below by the
Levine-Tristram signatures, and differs from the standard fusion number by an
arbitrarily large amount.
