 For a very long time, unless you were using USB, a midi jack looked like this. The impressively substantial 5-pin DIN connector. Turns out the two outside pins on the old midi DIN connector weren't used, only three were needed, so manufacturers took it upon themselves to start using something smaller, namely the 3.5mm TRS jack. And of course, without an industry standard, for a while we had two standards, midi TRSA and midi TRSB, which look identical, but needed an adapter. Yeah, thankfully the Midi Manufacturers Association stepped in and chose midi TRS type A as the standard we have today. So just be aware that older devices using midi TRSB are still out there, waiting to confuse and frustrate.