After two days of closing in, on 17 July 1975 at 16:10 UTC the two spacecraft joined together internationally for the first time in history. The key was the androgynous APAS-75 docking system on the docking module — unlike earlier "male–female" couplings, both craft could use an identical, symmetrical mechanism.
The docking module also solved a fundamental technical problem: Apollo used a pure-oxygen atmosphere at low pressure, while Soyuz breathed a nitrogen–oxygen mix at close to sea-level pressure. A direct passage would risk decompression sickness, so the module worked as an airlock in which the crews equalised pressure between the ships.
NASA — The Apollo-Soyuz Mission · NASA NSSDCA — Apollo-Soyuz Test Project · Spaceline — ASTP Mission Fact Sheet — full list on Apollo–Soyuz