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Let $X$ be an $n$--element finite set, $0<k\leq n/2$ an integer. Suppose that$\{A_1,A_2\} $ and $\{B_1,B_2\} $ are pairs of disjoint $k$-element subsets of$X$ (that is, $|A_1|=|A_2|=|B_1|=|B_2|=k$, $A_1\cap A_2=\emptyset$, $B_1\capB_2=\emptyset$). Define the distance of these pairs by $d(\{A_1,A_2\},\{B_1,B_2\})=\min \{|A_1-B_1|+|A_2-B_2|, |A_1-B_2|+|A_2-B_1|\} $. This is theminimum number of elements of $A_1\cup A_2$ one has to move to obtain the otherpair $\{B_1,B_2\}$. Let $C(n,k,d)$ be the maximum size of a family of pairs ofdisjoint subsets, such that the distance of any two pairs is at least $d$.  Here we establish a conjecture of Brightwell and Katona concerning anasymptotic formula for $C(n,k,d)$ for $k,d$ are fixed and $n\to \infty$. Also,we find the exact value of $C(n,k,d)$ in an infinite number of cases, by usingspecial difference sets of integers. Finally, the questions discussed above areput into a more general context and a number of coding theory type problems areproposed.

A coding problem for pairs of subsets