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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     The invention relates to suspended ceiling systems and, in particular, to a novel yoke for suspending a pair of main runners in parallel relation. 
     PRIOR ART 
     Certain ceiling treatments or designs utilize main runners or tees in relatively closely spaced pairs to give a ceiling a distinctive appearance and/or to provide an intermediate space for lighting, HVAC systems, sprinkler systems, and like services. It is known to use inverted U-shaped brackets or yokes to support a pair of main runners in close parallel relation. Such brackets, typically, are suspended in the customary manner that the remainder of the suspension grid is carried. Prior art yoke designs involve somewhat tedious installation procedures and/or are limited to single channel widths. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved yoke for spacing main runners in a suspended ceiling grid to form relatively narrow utility channels. The yoke includes mounting tabs that are arranged to be received in the cross runner slots existing in the main runners. The mounting tabs afford quick initial mounting of the main runners without tools, clamps and other instrumentalities. The yoke includes a catch that allows a runner to be received on the tab when the runner is twisted about its length and resists release of the runner from the tab when the runner is allowed to assume a normal orientation. 
     In its preferred embodiment, the yoke is assembled from two identical pieces. The pieces are lapped and screwed together by the installer. The two piece construction enables the yoke to be adjustable to the desired channel width between the main runners. The disclosed yoke, besides simplifying the installation of main runners for the tradesman, offers benefits to a manufacturer, distributors and contractors. One yoke fits commonly used utility channel sizes. Thus, inventory is greatly simplified for those engaged in the supply chain or is a contractor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a yoke of the invention shown assembled with local areas of a pair of parallel main runners; 
         FIG. 1A  is a fragmentary view similar to  FIG. 1  showing certain details of the yoke and main runner assembly; 
         FIG. 2  is an elevational view of one part of the yoke; and 
         FIG. 3  is an edge view of the one part of the yoke. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A yoke  10  shown in  FIG. 1  supports a pair of main grid runners  11  at a desired spacing to provide a relatively narrow utility channel between the runners. The channel formed between the runners  11  can be used to integrate lights, air vents, speakers, and other devices or services into a grid of like main and cross runners of a suspended ceiling. The illustrated grid runners  11  are of the familiar inverted tee cross section, but can have other known cross sectional shapes. The channel width is relatively narrow compared to the normal spacing used with main runners. 
     The yoke  10  is used with additional identical yokes spaced along the length of the main runners  11  which length, typically, is 12 ft. The yoke  10  has the general shape of an inverted U with an upper horizontal span  16  and depending legs  17  adjacent the ends of the span. As later indicated, the spacing between the legs  17  determines the relative spacing between the main runners  11 . 
     Preferably the yoke  10  is an assembly of two identical parts  21 . A part  21  is generally L-shaped, i.e. its shape is primarily that of a right angle. Ideally, the part  21  is stamped of sheet metal, usually galvanized steel. A major portion of the part  21  remains planar. An upper arm  22  of the part  21  is generally flat, apart from a narrow flange  23  along its upper edge at a right angle to the plane of the arm proper. A parallel series of spaced small and large holes  24 ,  25  are punched along the length of the arm  22 . 
     Adjacent a lower end  26 , the leg  17  includes a tab  27  projecting in parallel to the overlying arm  22 . At a vertical line  28  where the tab  27  merges with the lower end  26  of the leg  17 , a hole  29  is stamped to weaken the part for purposes of enabling and controlling manual bending of the tab as will be discussed. The tab  27  has a vertical height and a thickness enabling it to be received in a cross runner slot  31  in a web  32  of a main runner  11  while leaving sufficient room in the slot for a connector  33  of a cross runner  34 . The leg  17 , above the tab  27 , has an offset portion  36  that lies in a vertical plane horizontally displaced from the plane of a major part of the leg that exists above this portion. It will be seen from  FIG. 3  that the tab  27  lies in a plane parallel and immediately adjacent the plane of the upper part of the leg  17 . The offset of the portion  36  from the plane of the upper leg portion corresponds to half the width of a reinforcing bulb  37  of a cross runner  34 . This offset geometry allows the leg  17  to support a cross runner  34  at a central or medial vertical plane of the cross runner thereby assuring that these elements tend to hang vertically. The lower end  26  of the leg  17  is adapted to abut a web  38  of a cross runner  34  and has a hole  39  for receiving a screw to fasten this end to the abutting web. 
     Spaced above the tab  27 , the leg  17  has a portion bent into a transverse plane and forming a catch or hook  41 . A vertical edge of the leg  17  is bent to form a small stiffening flange  42 . 
       FIG. 1  illustrates a pair of the right angle parts  21  held by lapping their respective upper arms  22  and assembled in position by driving screws  46  or inserting rivets in the preformed small holes  24 . The holes  24  are located in their respective parts to permit adjustment to a standard channel width when a hole or holes of one part  21  are registered with the hole or holes of the other part  21 . The thus assembled yoke  10  is configured to support a pair of main runners  11  at a space corresponding to a standard channel width of, for example, 2 in., 4 in., 6 in., and 8 in. Ordinarily, a pair of main runners  11  will be supported by several yokes  10  spaced along the length of the runners. An installer, typically, will assemble the required number of yokes  10  each adjusted to a specific channel width and hang them with suspension wires  48  from overhead structure. The larger holes  25  are provided to receive the suspension wires  48 . Ideally, the holes  25  are large enough to accept multiple wires where splay wire type applications are desired or required such as in seismic applications. 
     A main runner  11  is installed on a yoke  10  by holding the runner between the legs  17  and twisting it about its longitudinal axis so that the reinforcing bulb, designated  51  at the top of the runner is tilted toward the side of the yoke to which it is to be mounted. With the runner  11  tilted, the bulb  51  is slipped under the catch  41 . With a cross runner slot  31 , aligned with a tab  27 , the runner is turned upright so that the tab enters and extends through the cross runner slot. At this stage, at least the local section of the main runner  11  is fully supported on the respective yoke  10 . It will be appreciated that this mounting is accomplished without the use of tools or fixtures. The tab  27  can be manually bent back against the web  32  of the main runner  11  (to the left in  FIG. 1A ) immediately or after tabs  27  of other yokes  10  are deployed in respective cross runner slots  31 . 
     Once a main runner  11  is installed on a number of yokes  10 , cross runners can be assembled on it. The other main runner  11  to be assembled on a yoke can be installed before or after cross runners  34  are installed on the first placed main runner  11 . At some point, typically before any nearby ceiling panels are installed, the tabs  27  can be secured to main runner webs  32  with screws run through tab holes  53 . Likewise, screws can fix the legs  17  to the cross runner webs  38  at the provided holes  39  and to the cross runner reinforcing bulb at selected ones of the provided holes, designated  54 . 
     When the yokes  10  associated with a pair of main runners  11  are secured to the main runner webs  32  and the cross runner webs  38  and reinforcing bulbs  37  with fasteners in the provided holes, a relatively rigid structure results. The rigidity is very helpful to the installer when uneven loads are imposed on a suspended grid and forces tend to distort it out of a plane. This phenomena, when using prior art yokes, hindered the installer and often required him to reset the grid. The slim profile of the yoke  10 , seen in  FIG. 3 , reduces potential interference with components to be carried in the channel formed by the main runners  11 . 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

Summary:
A sheet metal yoke for supporting a pair of main runners of a suspended ceiling grid in parallel alignment with a relatively narrow spacing, the yoke having an inverted U-shape with a horizontal span and a depending leg adjacent each end of the span, a lower end of each leg having a tab extending towards the other leg, the tab being proportioned to fit in a cross runner slot of a main runner and being capable of vertically supporting the main runner at a desired elevation.