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The German manual of my washer-dryer says it needs a water connection with measurements specified as 26,4 mm = 3/4". Assuming those quotation marks were meant to be prime marks, i.e. stand for inches, I ordered a 3/4 inch manometer. Turns out the 3/4 inches of the manometer were "actual" inches (i.e. 19.1 mm), whereas the 26.4 mm of my washer-dryer were 1,04 "actual" inches, not 0.75. If not "actual" inches, what does 3/4" refer to? I did some research on local home improvement store websites, where I found pipes labelled as both R 3/4 and 3/4".

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    Pipe measurements are for the inside diameter of the pipe. Tubing is usually outside diameter measurements. – Ed Beal Apr 3 at 22:19
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They use a hose thread the German one specified is a British Standard Pipe(BSP) thread that is used, which is ​3⁄4 inch and 14 TPI (male part outside diameter is 26.441 mm or 1.04 in).

In the USA, its territories and Canada, the standard thread used for garden hoses is commonly known as garden hose thread (GHT), but officially its title is NHR ('NH' standing for "National Hose"). There are two designations is for thin-walled couplers on hoses formed from thin walled material, and NH is for full-form threads on thicker material used to make valves and fittings, produced by cutting or rolling Thread pitch is 11.5 threads per inch (TI). All fittings are the same size, but specified by pipe or hose's internal dimensions. So typical fittings are 3/4-11.5NH or 5/8-11.5NH (3/4 is the internal diameter, 11.5 is the thread pitch) GHT is not compatible with National Pipe Tapered (NPT).For the rest of the world, the British Standard Pipe Thread standard (BSP) is used. Domestic fittings are 3/4 inch BSP and thread pitch is 14 threads per inch. Taps used on farms, factories and in horticulture and fittings for garden irrigation pumps use a 1 inch BSP thread system.

This info is extracted from the guide: The Complete Guide to Garden Hose Fittings

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