s has also been termed an atrial gallop or a presystolic gallop because of its occurrence late in the heart cycle. it is a type of gallop rhythm by virtue of having an extra sound; the other gallop rhythm is called s. the two are quite different, but they may sometimes occur together forming a quadruple gallop. if the heart rate is also very fast (tachycardia), it can become difficult to distinguish between s and s thus producing a single sound called a summation gallop.