the securing apertures 40 of the base plate 24 by positioning of selected apertures 46 of the sliding adjustment plate 26 with respect to the securing apertures 40 each aperture 46 preferably is threaded to receive and engage the fastener 42 inserted through the cooperating securing aperture 40 adjacent thereto to vary the distance the end 44 of the adjustment plate 26 extends outwardly from the recessed bed 36 as fig2 generally illustrates the apertures 46 are provided in a predetermined spaced apart arrangement seen in fig2 and 4 the spaced apart arrangement of the apertures 46 permits ready adjustment of the length of the support base 18 by the selective alignment of the appropriate securing apertures 40 and 46 to position the end 44 of the adjustment plate 26 at a desired location for example as indicated generally by arrow 34 b 34 or in the alternative positions illustrated in phantom outline by the arrows 34 c 34 or 34 d 34 the positions indicated by arrows 34 b 34 34 c 34 and 34 d 34 seen in fig2 are governed by the particular apertures 46 in registry with the securing apertures 40 as well as the particular spaced apart arrangement provided it is recognized that the range of adjustment of the length of the support base 18 may be varied for example by varying the lengths of the adjustment plate 26 and the recessed bed 36 the number and spaced apart arrangement of the apertures 46 or the method of securing the adjustment plate 26 relative to the recessed bed 36 to anchor the chair 10 relative to the fixed.