By multiplying these modules into a grid, the structure could be extended virtually infinitely. In its original form, the ground level of the Crystal Palace (in plan) measured by , which equates to a grid 77 modules long by 19 modules wide. Because each module was self-supporting, Paxton was able to leave out modules in some areas, creating larger square or rectangular spaces within the building to accommodate larger exhibits. On the lower level, these larger spaces were covered by the floor above, and on the upper level by longer spans of roofing, but the dimensions of these larger spaces were always multiples of the basic by grid unit. The modules were also strong enough to be stacked vertically, enabling Paxton to add an upper floor that nearly doubled the amount of available exhibition space. Paxton also used longer trellis girders to create a clear span for the roof of the immense central gallery, which was wide and long.