Why is the fertilization characteristic of angiosperms called double?

This is a sexual process in angiosperms, in which both the egg and the central cell of the embryo sac are fertilized. In double fertilization, both sperm are involved, brought into the embryonic sac by the pollen tube; the nucleus of one sperm merges with the nucleus of the egg, the nucleus of the second – with the polar nuclei or with the secondary nucleus of the embryo sac. The embryo develops from the fertilized egg, and the endosperm from the central cell.