Stabilizer, also known as Isocyanuric Acid. Helps prevent the loss of chlorine to the sun. Also helps prolong shelf life. If stabilized form of chlorine is used regularly, it only needs to be added to new water. (See also 'Sodium Dichloro' and 'Sodium Trichloro') Eventually the turbity will increase to high levels with this method of use but it only slows down the action of the chlorine and does not cause scaling, staining, or rock formations in filter like calcium based products will. (See also "Stressed Water"). IMPORTANT NOTE: unstabilized chlorine burns out of an exposed (to the sun and elements) pool at a rate of about 1 p.p.m. per hour and a shelf life that decreases rapidly. See also 'Calcium Hypochloryte'