The refractometers which are the subject of this Recommendation are instruments which measure the refractive index of grape must before fermentation, using the phenomenon of light refraction or of total internal reflection of light. The result of measurement is given in one of the forms indicated in section III below. These instruments may also be used to measure the refractive index of sugar solutions and, possibly, of concentrated must. This Recommendation does not deal with instruments intended for rectified concentrated must, nor does it intend to impede the development of new technologies. Refractometers are classified in three types (see clause 2). National regulations may specify the conditions under which the various types of refractometers may be used, and even prohibit the use of given types in certain locations or for certain purposes.