be opened the vent stop device typically is positioned in such manner as to limit opening movement of a window sash to a desired maximum typically about 4 inches in the illustrated form of the invention the retracted limit position of the tumbler 22 is determined in part by means of an extension 38 of the bottom 39 of the tumbler as shown in fig5 the extension 38 has a bottom surface 40 that engages the bottom wall 28 to limit pivotal retracting movement of the tumbler once in the retracted position the tumbler is held in that position by a unique detent means to be described the extension 38 extends a short distance beyond the abutment surface 37 and provides an outer surface 41 that can function as a secondary stop feature if forces greater than normal were to be applied to a sash in an opening direction the extension surface 41 will engage the face of the sash and assist in maintaining its position as shown particularly in fig6 the short end 36 of the tumbler which is recessed when the tumbler is in its extended limit position has downwardly facing surfaces 42 43 which engage with upwardly facing surfaces 44 45 of the housing bottom wall to form a strong positive limit stop to pivoting movement of the tumbler in the extending direction this is arranged to strongly resist pivoting movement of the tumbler in response to sash pressure on the abutment surface 37 additionally the recessed end 36 of the tumbler when the tumbler is in its extended position has a surface portion 46 which bears against the inner surface of the end wall 26 this serves to transmit longitudinally directed forces applied against the tumbler via the abutment surface 37 into the housing end.