Grandfather clocks and grandmother clocks look very similar, although some sources say that a clock over six feet, three inches is by definition the more masculine of the two and that the more feminine tends to be between five feet, four inches and five feet, nine inches. Classifying clocks is, however, largely up to individual clock merchants themselves. Some clocks are spring-driven, and that causes some to think of it as feminine based largely on that, while considering weight-driven clocks to be of the masculine variety.