What are three grades of silk explain?

'''Grade 1''' is silk yarn reeled from the inner layers of the cocoon. It is even in structure, size and color, round, clean and free from adulterated materials. '''Grade 2''' is silk yarn reeled from a combination of the inner and outer layers of the cocoon. It is not even in structure, but is even in size and color, round, clean and free from adulterated materials. '''Grade 3''' is the silk yarn reeled from the outer layer of the cocoon with the addition of the floss from the innermost layer. It may contain some knots. It is round, even in size and color, clean and free from adulterated materials.