outlying portions 20 of track 10 are positioned so that one of said portions overlies the other as depicted this aligns bores 22 spacer 30 is then positioed between mullion 24 and the innermost track 10 and an elongate screw 26a is positioned through the aligned bores 22 and screwed into the mullion as depicted this mounting covers mullion 24 from view the space between guide portions 18 of the tracks 10 and the surface of mullion 24 accommodates the wheel members of the novel assembly referring now to fig5 it will there be seen that the wheel housing is denoted 32 as a whole housing 32 houses three rotatably mounted wheel members 34 36 38 all of which are shown in phantom lines because they are positioned on the 34 hidden 34 side of such housing 32 in the view of fig5 actually only the hubs of the wheels are depicted in fig5 and 6 to simplify the drawings wheel members 34 36 are positioned in a vertical plane when operatively deployed and rollingly engage portion 18 of track 10 wheel member 38 is positioned on the visible side of track 10 however and is preferably hidden from view by opaque cover 40 shown in fig1 6 and 7 cover 40 is integrally formed with housing 32 housing 32 also includes a pair of outwardly turned covers 42 44 for wheel members 34 36 respectively the term 34 outwardly 34 is used because the direction 34 inwardly 34 will hereinafter be used to indicate the direction toward the center of a window covering i e the term 34 outwardly 34 refers to the direction away from the center of the covering or to the direction of the.