Mensur is the ratio of two length specifications that are critical to playing the instrument. The first length is called the f-stop or body stop; it refers to the distance from the top edge of the belly to the f-hole notches. The second, the neck stop, specifies the length of the neck. These two measures determine the vibrating string length. Until the 18th century, violin necks were generally somewhat shorter than today, and their length was not standardized. From about 1900, slightly longer, angled-back necks prevailed, allowing more pressure to be put on the bridge, thus increasing sound volume. The ratio between the neck stop and the f-stop is essential for musicians as a guidepost to correct finger positions on the fingerboard.