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Where do you measure the bore of a trombone?

I usually measure the inside diameter of the fitting on the slide where it connects to the bell section. If you measure the inside of the 'socks'... the ID is going to be a tad larger. If you measure where the mouthpiece goes, you will really be measuring the inside of the venturi.

Thanks to all who responded.

Measure both inner slides at the bottom end. Some are dual bore with the top slide smaller. I have never seen one where it was larger there, except maybe a cheap student horn that could be inaccurate anywhere.

Typical dual bore horns are most models of Olds, King 2B, and Bach 16.

Some other possible smaller bores:
King 2B .481 - .491 dual bore
Bach 6 .485 both tubes
You might run into a .495, .510, .515, or .520 especially on Olds trombones

Yet another bore size, Amati bass trombones .551 .

Still more.................My Besson 943 bass is .563 and my Olds O-25 is .555

Isn't .562/3 is just rounding of the actual size (.5625ish? no idea). I recall reading about people with ".563" Yamahas swearing that they could notice the difference.
There is/was a Holton tenor with a .559 slide tube somewhere. I've always wanted to try those Holton tweeners, especially the 159- dual bore slide, 9" bell, 2 slides in the valve tubing, usable thumb brace,...

My old Kanstul contra had unusual slide proportions- .562/94 slide, .620 and .650 valves. I always thought it'd play a bit better with a .578/94 slide. I guess Kanstul didn't want to make .578 slides, even though they could offer them on bass trombones then.

I never measured my Besson. .563 is what the catalogue said. I thought it was just to be able to say it was bigger than................................If you can convince me that you can tell the difference I'll kiss your..........................................................................................................girl friend.

Usually we try to weed them out beforehand but the trombone bore can certainly be measured in the drawing room whilst partaking of sherry and cigars.