It appears from such an example as that in § 40 (c) that E. distich is co-radicate with (i. e. from the same root as) E. stair. The former is a borrowed word, from the weak grade, and the latter a native word, from the second grade of the same root *steig-. A similar relationship usually holds even in cases where the root does not appear as a known verb; and we may go so far as to consider words as co-radicate, provided that the consonants of the root are the same (such as st . g) and the vowels are regularly related by gradation.
—W. W. Skeat, 1905, A Primer of Classical and English Philology