1. A HINGE BRACKET FOR CONNECTING THE ADJACENT ENDS OF TWO LONGITUDINAL FRAME MEMBERS TOGETHER AND TO AN END OF A THIRD MEMBER WHILE PERMITTING SWINGING MOVEMENT OF SAID TWO MEMBERS WITH RESPECT TO SAID THIRD MEMBER BETWEEN A FIRST POSITION IN WHICH THEY EXTEND IN LONGITUDINAL ALIGNMENT AND AT A RIGHT ANGLE TO SAID THIRD MEMBER AND A SECOND POSITION IN WHICH THEY ARE DISPOSED SUBSTANTIALLY PARALLEL TO ONE ANOTHER AND TO SAID THIRD MEMBER THEN DISPOSED INTERMEDIATE THEM, SAID BRACKET BEING OF ONE-PIECE CONSTRUCTION AND HAVING T-CONFIGURATION WHEN VIEWED IN SIDE ELEVATION AND COMPRISING A HORIZONTAL TOP BAR AND A VERTICAL STEM DEPENDING FROM SAID TOP BAR MIDWAY OF THE ENDS OF THE LATTER, SAID TOP BAR BEING DEFINED BY A PAIR OF ELONGATE, PARALLEL, UPRIGHT FLANGES SECURED TO ONE ANOTHER IN LATERALLY SPACED RELATION AT THEIR MID PORTIONS BY A SHORT-LENGTH FLAT CONNECTING WEB, SAID VERTICAL STEM BEING DEFINED BY SPACED-APART LEG PORTIONS DISPOSED TRANSVERSELY OF SAID UPRIGHT FLANGES AND DEPENDING FROM THE END EDGES OF SAID CONNECTING FLANGE, SAID UPRIGHT FLANGES ADAPTED TO RECEIVE BETWEEN THEM THE ADJACENT END PORTIONS OF SAID TWO MEMBERS AND BEING PROVIDED WITH MEANS ADJACENT THEIR OUTER ENDS FOR RECEIVING PIVOT MEANS FOR PIVOTALLY CONNECTING SAID ADJACENT END PORTIONS OF THE TWO MEMBERS THERETO AND THEREBY TO THE OUTER ENDS OF THE BRACKET TOP BAR, THE LEG PORTIONS DEFINING THE VERTICAL STEM BEING SHAPED TO THE CROSS-SECTIONAL CONFIGURATION OF THE THIRD MEMBER AND BEING SPACED APART A DISTANCE AS TO RECEIVE AT LEAST THE UPPER PORTION OF SAID THIRD MEMBER WITH TIGHT FIT, AND SAID LEG PORTIONS BEING PROVIDED INTERMEDIATE THEIR LENGTH WITH MEANS FOR RECEIVING A RIVET FOR FIXEDLY SECURING THE THIRD MEMBER IN PLACE BETWEEN SAID LEG PORTIONS, THE CONNECTING WEB PROVIDING A REST SURFACE FOR THE IMMEDIATELY ADJACENT ENDS OF SAID TWO MEMBERS WHEN THE LATTER ARE IN THEIR FIRST POSITION AFORESAID AND FURTHER PROVIDING AN ABUTMENT SURFACE AGAINST WHICH THE END EDGE OF THE SECURED THIRD MEMBER IS ADAPTED TO ENGAGE.