I can get the vector representation of helices. However if I calculate the arccos of these 2 vector's inner product, I can only get the angle from 0 to 180 degree. I see many paper using tableau representation calculate the angles from -180 to 180 degrees. One of the earlies paper had a sentence: "its sign is defined by the convention that a clockwise rotation of the nearer vector with respect to the farther is positive". But I don't know what does it mean. Anyone can help?