[topazas]

Awesome, but what does 1'' mean? Some weird measurement unit?

[hnuser123456]

That would be 2.54 times 1/100th of the distance light travels in a vacuum during 1/299,792,458 of the time it takes for 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a cesium-133 atom, if you prefer.

[bluGill]

Wrong!

While there is a 1 inch measure in common use that is as you described, the subject here is EMT. There is no dimension in EMT that is 1 inch by the system you describe. The diameter is close to 1 inch, but it is noticeably different to the naked eye, and for all useful purposes different enough that anything actually 1 inch in diameter is not compatible.

[bluGill]

27 mm outside diameter. Which has zero relation to any other inch you might have heard of in common use (ie in the US).

[LukeShu]

" is imperial inches, ' is imperial feet.

[unwind]

It's awesome that they combine 1" pipe with a 5 mm hex [1] (often "Allen" in the US) fastener. :)

[1]: https://en.wikipedia.org/wiki/Hex_key

[gibspaulding]

You see a lot of this in the bicycle industry. There are a lot of older standards in use like 9/16” pedal threads, 1 1/8” steerer tubes or 1” (25.4mm) handlebars but any new standard is metric - so bottom brackets, wheels, newer seat post diameters are all metric. It can make for some very strange looking spec sheets.

[bluGill]

The actual size is 27mm. If you convert to an imperial system the size is not a nice number.

[wezdog1]

/s might have been needed, it appears.

[Modified3019]

1" is one inch

1' is one foot

So yeah, a weird measurement unit. Technically the symbol to be used is a prime symbol (https://en.m.wikipedia.org/wiki/Prime_(symbol)), but what’s used in practice is anything that looks close enough.