A blade element pair (20) for a refiner (1, 2, 3) is intended for refining fibrous material. Each blade element (5, 5′, 8, 8′) of the blade element pair (20) has a refining surface (6, 9) with blade bars (16, 18) and blade grooves (17, 19) extending along the blade element (5, 5′, 8, 8′), and openings (14,15) extending through the blade element (5, 5′, 8, 8′). The openings (14, 15) in one of the blade elements (5, 5′, 8, 8′) is at different axial (A) or radial (R) positions from the openings (14,15) in the other blade element (5, 5′, 8, 8′) when the blade elements (5, 5′, 8, 8′) of the blade element pair (20) are set substantially opposite to each other.