A term formerly used in a First-to-Invent patent priority system as prevailed in the United States. In such a system a prior filing or prior art that predated the presumptive Priority Date of a patent application or patent could preclude grant of a patent. One solution was for the inventor or inventors to provide an affidavit that their Reduction to Practice of the claimed invention predated the prior reference – since the affidavit was under oath this was described as ‘swearing behind’ or ‘to swear behind’ the prior reference.