The first British railway semaphore signals had arms that could be worked to three positions, in the lower quadrant. Used in conjunction with the time-interval system, the arm horizontal meant "danger", inclined downwards at 45 degrees meant "caution" and the arm vertical (arm hidden within the post) meant "clear". The vertical indication gradually came to be discontinued as the absolute block system superseded time-interval working. The Great Northern Railway was the first company to introduce "somersault" signals, mounted away from the post, after an accident in January 1876 when a train passed a signal giving a false "clear" aspect because the signal arm had frozen into its slot during a blizzard.
