Long recombination distances are, in general, not simply additive. Partly, this is because the maximum RF is 50cM, but this is also because of double crossovers . A single crossover event between fo and B will produce recombinant progeny which, when counted, will add to the recombination frequency. Therefore, the number of double recombinants reduces the apparent recombination frequency for distant markers. Because of this, recombination frequencies are not simply additive.
Double crossovers are very useful for determining map order.