Crossing over, in addition to its strictly genetic role, is responsible for an orderly reduction of the chromosome number. As such, it is strictly controlled in frequency and distribution. In our new paper, we described direct evidence of chromatid interference. Using in situ probing in two interspecific plant hybrids during anaphase I, we demonstrated that the involvement of four chromatids in double crossovers is significantly more frequent than expected (64% versus 25%). We also provided a physical measure of the crossover interference distance, covering ~30–40% of the relative chromosome arm length, and showed that the two arms of a chromosome appear to act as independent units in the process of crossing over.