Tired of biters swarming through your gates and breaking your things while you're beating a hasty retreat? The first gate outputs G and opens when G > H, the second outputs H and opens when H > G. That way both gates are never open at the same time, greatly reducing the number of natives that can follow you in. Be careful to position the gates far enough away or you won't be able to leave the airlock. Three tiles inbetween is the minimum for pedestrian traffic. Also, if they are too close you have to come to a complete stop before going through.
Since I started making these I've only ever had one bug follow me through. If I made them narrower it would probably be zero, but that looks ugly, so I consider the single bug an acceptable trade-off.