Back frame, mold for back frame, and backlight system

A back frame of a flat panel display device, wherein:

• the back frame comprises at least two primary assembling pieces including a first primary assembling piece and a second primary assembling piece, the at least two primary assembling pieces being connected through direct or indirect joining;
• wherein the at least two primary assembling piece, that are directly joined or indirectly joined are set in mutual rotation symmetry of a predetermined angle with respect to each others;
• wherein each of the first and second primary assembling pieces comprises two spaced joint sections and one of the joint sections of the second primary assembling piece is switchably jointed to a first one and a second one of the joint sections of the first primary assembling piece, with the other one of the joint sections of the first primary assembling piece being idle, so that the second primary assembling piece is connected to the first primary assembling piece in a position-adjustable manner to respectively define a first size and a second size of the back frame, the second size being different from the first size.