The typical aviation headset features two cords with connectors that at first glance are identical. One is a standard 1/4" (0.25 inch) stereo audio jack.
What kind of connector is the slightly smaller, slightly narrower one?
It's for the microphone (the standard sized one is for the head phones). They are different size so the pilot knows to try the other hole if it doesn't fit.
Commercial and general aviation (GA) civil airplane headset plugs are similar, but not identical. A standard 1⁄4 in monaural plug, type PJ-055, is used for headphones, paired with special tip-ring-sleeve, 0.206 inch diameter plug, type PJ-068, for the microphone. On the microphone plug the Ring is used for the microphone 'hot' and the sleeve is common or microphone 'Lo'.