It's the point at which our two chromatids, this chromatid and this chromatid, basically overlap. Now, because we have four individual chromatids, we have one chromatid, two, three chromatid, four. This entire structure is known as a tetrid. So basically, the tetrid refers to the orientation of these individual four chromatids that are very close with respect to one another. Now, once synapses actually takes place and we form our chiasma, then crossing over actually takes place. So crossing over is a type of genetic recombination process in which we have the exchange of genetic information from this chromatid and this chromatid.