Sentence:  When now we count the frequency of the 71 items in the two language families appearing in the same column or grade , or one column or grade apart , or two or three or four , we find these differences : * * f This distribution can be summarized by averaging the distance in grades apart : * * f ; which , divided by * * f gives a mean of 1.07 <WSD>grades</WSD> apart . 
Meaning of the word marked with <WSD> tags:  a body of students who are taught together