They matched the cord to some sold at McGuckin's Hardware in Boulder, CO.
It's a special type of utility cord that is flat instead of rounded like most cord. That assures that it doesn't slip.
The fibers found entangled in the cord knot, along with JonBenet's hair, were red ones consistent with the sweater Patsy had had on that night and the next morning - the same red fibers found on the sticky side of the duct tape on JonBenet's mouth.
Fibers can only be called "consistent" microscopically in a legal setting, not "matched," because of the commonality. However, when you see the microscopic close-ups, the "match" becomes obvious when an expert witness is on the stand. They also compare fabric type, dye batch, thread count, length, and so on.
but no other length of the same cord, or the duct tape roll, was found in the house, right?
so that means the killer brought those items into the house with them, or they were disposed of from inside the house.
and I don't know how they would explain the fibers in the case against Karr.