A letter matrix is an n-by-n matrix whose entries are n symbols, each appearing n times. The row (column) distribution of a letter matrix is an n-by-n nonnegative integer matrix that tells how many of each letter are in each row (column). A row distribution R and a column distribution C are compatible if there exits a letter matrix A whose row distribution is R and whose column distribution is C. We show that the matrix J of all ones is compatible with any C, and we also consider the the problem of when R and C pairs are compatible in terms of their values and patterns inside the distribution matrices
