It is important to keep the horizon parallel to edge of the photograph. The horizon level is easy to overlook when taking the photo but is easy to see when looking at it afterwards. Make sure that if you have a strong horizon level that you keep it level to the frame of the photo. Place the line one third of the way into the frame from the top or bottom (see Rule of Thirds).

Another example, if your photo is of a sunset, it is best if you place the horizon line on the bottom third of the frame so more of the sky is captured. Always keep the horizon level with the top or bottom of the viewfinder.