side of the machine in directions as indicated by arrows c each keyway is disposed immediately above a respective line of joinder between respective adjacent suction devices in practice the trim boards will be arranged at the desired distance inboard of the inside wall 11 of the headbox 10 as can be appreciated by reference to fig2 the trim boards can be moved laterally toward or away from the table rail r of the papermaking machine the trim boards function in the manner of sliding valves so that the suspension on the wire may be held back from flowing through the wire along the defined length and thereby not only stop its flow through the wire but also preclude the deposition of fibers along that length the term trim is normally used to mean cut but in papermaking the term trim is used to mean separating an edging from a main body portion of a web being formed cutting in such instance is not involved by the aligning of the trim boards along the axis of the movement of the wire a width of in the described exemplification 2 34 along the entire mat forming length is provided in which width drainage through the wire is precluded wherefor disposition of fibers is precluded mat formation being there prevented a split mat.