MANY papers dealing with the structure of interstratified minerals such as illite-montmorillonite, chlorite-mont-morillonite, chlorite-vermiculite, etc., have been published up to the present. In describing the structure of these minerals, the terms ‘random’ and ‘regular’ have been often used, but their definition is not very clear, since the term ‘regular’ has been applied mostly to ‘1:1 regular structure’, whereas the term ‘random’ has been used for structures other than regular. It is necessary to investigate in detail the structure of interstratified minerals.