And in almost every headphone connector since the 60's or so... That's how it used to switch the sound from the speakers to the headphones when you just plugged them in.

Cool, had wondered how old that feature was.

Side note; Not sure I've heard 3.5mm connectors called 1/8" connectors before, though I have heard of the larger 1/4" connectors.

They're not actually 1/8th of an Inch either.

And famously, a 2x4 piece of lumber is not actually 2" by 4" either.

Well, that depends. Prior to finish planing they are exactly 2" x 4", and that's where they get their name. You can buy these in bulk straight from the sawmill or lumber wholesalers. But most lumber is sold neatly planed and then the outside dimensions change.

TIL. I know that if you bought a 2x4 that was actually 2" x 4", it wouldn't fit into anything else that expects typical dimensions.

But it is in fact commonly called a 1/8" (headphone) jack.

As noted in my comment, never noticed 1/8" before (and am American, so no particular metric bias...). All e.g. MP3 players historically called them 3.5mm jacks, is what my brain is telling me.

Some googling:

3.5mm jack - 49million results

1/8" jack - 63 million results

3.5mm stereo jack - 19 million results

1/8 stereo jack 5.9 million

1/8" stereo jack - 5.47 million

Evidently, it's a common term! Guess once I decided what I was calling them, that's what I searched for, and what I skimmed technical details for.

Ammunition caliber is similarly interesting

Right .223 Remington has the same bullet diameter of .222 Remington; namely 0.224 inches.

Metric dimensions of rifle-cartridges, however, seem to reliably be the land-diameter: 5.56x45, 7.62x51, and 7.62x39 all have eponymous land-diameters.

Ok, I went into a on-off rabbithole this afternoon due to mistaking your comment as a claim that the 3.5mm jack had microphone as a standard in the 60's. Findings:

* Regarding "Jack" In February 1884, C. E. Scribner was issued US Patent 293,198 for a "jack-knife" connector that is the origin of calling the receptacle a "jack". . . . The current form of the switchboard-plug was patented prior to 1902 . . . It is today still used on mainstream musical equipment, especially on electric guitars. [0]

* 3.5mm connector There are at least four different varieties of 3.5 mm plugs and jacks, each with its own purpose and application. . . . TS, TRS, TRRS . . . The letter T stands for Tip, and the R stands for Ring (like a ring on your finger, not like ringing the telephone). . . . a TS plug has two conductors, a TRS has three, a TRRS has four, and a TRRRS has five. . . A TRRS or Tip Ring Ring Sleeve plug has four conductors and is very popular with 3.5mm, and can be used with stereo unbalanced audio with video… or with stereo unbalanced audio plus a mono microphone conductor. . . . Unfortunately, there are two conflicting standards associated with its use with stereo unbalanced audio plus a mono microphone conductor. [1]

0. https://en.wikipedia.org/wiki/Phone_connector_(audio)

1. https://www.provideocoalition.com/ts-trs-trrs-trrrs-combatin...

No, the older ones are much larger, and still in common use in audio studios and for musical instrument connections. They are exactly the same form factor as the ones used for patching and routing phone calls on the old switchboards, as your comment lists, and the addition of switches behind and next to the plug to be operated by the insertion of the plug is a pretty old practice as well. I've never seen a TRRS or a TRRRS in 6.3 mm but in 3.5 TRRS is pretty commonly used for the headset connector (and, just like in the days of old, when you plug it in the sense switch will disable the speaker and mic on the phone and route the signals through the plug instead).

I still have an old phone (not a smartphone) that uses a 3.5 mm TRRS jack for that purpose and I would not trade it for any Bluetooth gizmo, it works perfectly, you just need to be a bit careful with the cable. But no stress about the earpods running out of juice.

The first time I saw a 3.5 mm was in my 1975 six transistor Japanese little 9V AM radio. It came with one of those weird pinkish earpieces.

Can you recommend a 3.5mm TRRS headset that doesn't suck? All the ones I've tried have a terrible in-line mic that I think is supposed to work as a Lavalier, but in fact makes it sound like I have several layers of cloth over my mouth.

I use an original Nokia headset (the old ones from before the crash-and-burn), it works very well. Finding them could be an issue though, I got mine from the junk bin at one of those phone repair places, it was brand new and still bagged.

I'll keep an eye out for more if you want.

Thanks for the offer, but I think I'm on a different continent from you (I'm in California).

Ah yes, you are, I'm in Europe. The one I have is called the HS-45, the multimedia controls don't work with my current phone but the basics (mic, headphones, break connection and volume) do.

Microphone/headphone combos with a single TRRS jack are fundamentally going to be poor, because the microphone line isn’t balanced. Microphone signals are very low level and pickup noise easily.

You might get better luck with mic-in-cable jacks, my headphones came with one (TRS plug on headphone side, TRRS on other) but they can be bough as separate items.