engagement with flange 18 of the sink structure whereby the latter may be leveled and clamped tightly against the underside of the countertop 10 as indicated in fig4 of the drawings it also is to be noted that the two side walls 31 32 are parallel and the bottom side of the rail 20 is open with no intervening wall in the zone between the bottom portions of walls 31 and 32 for reasons which will be clarified hereinafter see fig9 while the two rails 20 serve to hold the adjustable leveling bolts and cooperating nuts for the purpose of clamping the rim of the sink tightly to the underside of the overlying countertop as above described it is essential for the successful operation of this operation that the rails be amply supported and held level in stationary spaced parallel relationship on opposite sides of the sink so that the upward threading action of the bolts 33 serve to move the sink rim uniformly into engagement with the countertop this function is accomplished by means of the mounting brackets 21 which are engaged with the opposite ends of each of the rails 20 to provide a reactionary platform against which the forces exerted by the leveling bolts are opposed as shown in fig7 and 8 each of the brackets 21 comprises the elongated leg.