are made of plastic and of a wide variety of diameters. Higher quality pieces are made of fiberglass, having a diameter of typically between 35 mm and 50 mm, and a height usually no greater than 8 mm. Nowadays, in formal games and tournaments, all buttons should be of the same material, color and size. Such standardization is decried by older players, as in their view it robs teams from variety and from similitude to real-life soccer teams. Buttons can be decorated with markings and numbers typical of a football jersey, as well as graphics of famous players or team logos, but no large diversity within a given team is allowed.. Goaltender pieces are rectangular blocks with dimensions and weights closely regulated. Typical dimension limits include 35 mm width x 80 mm height x 15 mm depth. The game ball is a sphere of woven felt with a diameter of 10 mm weighing between 0.1 and 0.3 grams. Buttons are propelled using a flat circular plastic disc (that could be a thin flat button, as well) of approximately the same diameter as the buttons or higher. Physical characteristics are not critical. Gameplay There are a variety of different rule systems to define play. The following general guidelines are common to most, if not all: At the opening kickoff, all 11 pieces are relegated to their respective sides. Field players are moved during the game by using a circular disk to exert a downward force on the field pieces, propelling them forward (pieces may never be pushed with the disk). Field pieces are propelled forward to strike the game ball. The ball is either sent forward and then hit again with the same field piece the next play (i.e. "dribbling"), sent forward towards another field piece on the same team (i.e. "passing"), or towards the opposing goal (i.e. "shooting"). Players alternate manipulating their pieces based on the possession rules defined in each rule system. Shots on goal must be declared by the attacking team prior to being taken to allow the defending team to adjust its goalkeeper. If the ball enters the goal without a shot having been declared, the goal is annulled. If a field piece strikes an opposing piece prior to hitting the ball, that is declared a foul. If the ball hits an opposing piece then stays within the field of play, possession is relinquished to the opposing