We study quadrilaterals Q which are given by two intervals on {~: Im ~ = 0} and {~: Im ~ = 1}, and two Jordan ares 7t, 3'z in {~: 0 < I m ~ < I} connecting these two intervals. Many practical problems require the determination of the module re(Q) of Q, but if Q is "long," i.e., if h ,= min{Re ~: ~ ~72} --max{Re ~: ~ E ~'1} is large, the conformal mapping of Q onto a rectangle becomes difficult because of the crowding effect. However, it turns out that re(Q) -h approaches a limit very quickly, as h ~ oo, and we can therefore estimate re(Q) -[m(Qt) + ra(Q2)] when Q is decomposed into two smaller quadrilaterals Q1, Q2. Several numerical examples are presented. This domain decomposition method goes back to Papamichael and Stylianopoulos [9]. On the other hand, experiments showed that re(Q) behaves rather regularly if Q is expanded horizontally. This caused Papamichael and Stylianopoulos [8], [9] to