Youden represented his designs as Latin rectangles; Fisher represented them as partial Latin squares. The entry in the ith row and jth column of Fisher's square is k if and only if the entry in the kth row and ith column of Youden's rectangle is j.
A Youden square is based on a square 2-design D. Each row of Youden's rectangle is a permutation of the numbers 1,2,...,n, while the columns are the blocks of D. If the blank and non-blank entries of Fisher's square are replaced by 0 and 1 respectively, we obtain the incidence matrix of D.
Here is an example, where the design is a 2-(7,3,1).