There are two common designs for easels: Tripod designs are based on three legs. Variations include crossbars to make the easel more stable and an independent mechanism to allow for the vertical adjustment of the working plane without sacrificing the stability of the three legs of the easel. H-Frame designs are based on right angles. All posts are generally parallel to each other with the base of the easel being rectangular. The main portion of the easel consists of two vertical posts with a horizontal crossbar support, thus giving the design the general shape of an "H." Variations include additions that allow the easel's verticality to be adjusted. There are three common usages for easels: Studio easels are meant for use in the artist's studio with limited need for the easel to be portable. Studio easels may be simple in design or very complex including winches, multiple masts and casters. The largest easels are studio easels with some being able to support weights of over 200 lb. and panels over 7 feet in height. Field easels are meant to be portable and for the creation of en plein air work. These easels are usually mid-sized or small, have telescopic or collapsible legs and are based on the tripod design. French box easels include a compartment in which to store art supplies conveniently along with a handle or straps so that the French box may be carried like a briefcase or a backpack. Display easels are meant for the display of finished works. These easels tend to be very simple in design with less concern for the stability needed by a working artist. Display easels can vary in size and sturdiness depending upon the weight and size of the object to be placed on them.