Superb photography and video'graphy accessory, especially helpful for landscape and architectural photographic applications. Works for both horizontal and vertical picture taking. Attaches to camera's flash hotshoe. Clear and easy to read!..Please Note: A Twin Axis spirit level provides assessment for correct horizontal positioning only - i.e. to make sure the horizon will be straight regardless of whether the camera is used in a horizontal or vertical fashion..One bubble is used for when the camera is placed horizontal on a tripod and the other bubble is used when turning the camera 90 degrees to take a picture vertically (like a tree or statue etc)..A triple axis spirit level would also confirm that the lens is not pointing upwards or downwards. Useful for taking several images together for panoramic stiching etc..Both models are very popular, so please think about which version would suit you best as you may not need or want to pay for the extra feature given by a 3 axis version..See the product links below for 2 axis version