  Two defect lines separated by a distance delta look from much larger
distances like a single defect. In the critical theory, when all scales are
large compared to the cutoff scale, this fusion of defect lines is universal.
We calculate the universal fusion rule in the critical 2D Ising model and show
that it is given by the Verlinde algebra of primary fields, combined with group
multiplication in O(1,1)/Z_2. Fusion is in general singular and requires the
subtraction of a divergent Casimir energy.
