displaying, on an electronic display device, a plurality of symbol-bearing reels comprising an array of symbol positions, the array of symbol positions comprising a plurality of rows and columns, each symbol position populated by a respective independent reel;

conducting a first spin of the symbol bearing reels to land symbols on the reels in the symbol positions of the array according to one or more first outputs of a continuously cycling random number generator;

in response to a triggering event, displaying, on the electronic display device, an animation of expansion of the array comprising expanding one or more of the rows of the array to add one or more symbol positions to each expanded row, wherein the expanded array is center aligned, and wherein the expanding includes expanding each of the one or more rows by a same number of symbol positions in opposite directions, and wherein any given row of the array can only expand if such expansion would cause that row to be no more than a predetermined number of positions larger than the number of symbol positions in any other row; and

conducting an additional spin of the symbol bearing reels to land symbols on the reels in the symbol positions of the expanded array according to one or more second outputs of the continuously cycling random number generator.