According to the International Skating Union (ISU), jumps must have the following characteristics to earn the most points: they must have "very good height and very good length"; they must be executed effortlessly, including the rhythm demonstrated during jump combinations; and they must have good take-offs and landings. The following are not required, but also taken into consideration: there must be steps executed before the beginning of the jump, or it must have either a creative or unexpected entry; the jump must match the music; and the skater must have, from the jump's take-off to its landing, a "very good body position". A jump combination, defined as "two (or more) jumps performed in immediate succession", is executed when a skater's landing foot of the first jump is also the take-off foot of the following jump. Jumps were viewed as "acrobatic tricks, not as a part of a skater’s art" and "had no place" in the skating practices in England during the 19th century, although skaters experimented with jumps from the ice during the last 25 years of the 1800s. Hops, or jumps without rotations, were done for safety reasons, to avoid obstacles, such as hats, barrels, and tree logs, on natural ice. In 1881, "Spuren Auf Dem Eise" ("Tracing on the Ice"), "a monumental publication describing the state of skating in Vienna", briefly mentioned jumps, describing three jumps in two pages. Jumping on skates was a part of the athletic side of free skating, and was considered inappropriate for female skaters.