  In the order picking problem, a picker has to collect a number of products in
a warehouse with a minimum length tour. Ratliff and Rosenthal gave a linear
algorithm solving the order picking problem in the case where the warehouse has
two cross aisles. Their algorithm allow the tour to double cross an entire
aisle. We prove that, in rectangular warehouses, there always exists a minimum
length tour which doesn't double cross an aisle.
