Length fast and Length slow are the terms most commonly used in optical mineralogy. When the fast ray vibrates more or less parallel to the length of an elongate mineral, then the elongation is called length fast. On the other hand, if the slow ray vibrates more or less parallel to the length of an elongate mineral, then the elongation is length slow. Length slow is also considered as positive elongation while length fast is the negative elongation. These signs of elongation are different from the optic signs.

            Sign of elongation is not always found in all minerals. Some minerals do not have elongation. The term does not clearly apply if the mineral does not have an elongate habit. On the other hand, in cases where the sign of elongation cannot be assigned, the vibration directions are approximately 45-degree to the length of the crystal.