The Mavic website gives rim specs as ETRTO compatible size, for example:

ETRTO compatible size: 559 x 21
Recommended nipple length: 12 mm
Recommended rim tape: 559 x 23 x 0.6
Recommended tire widths: 2.30 to 3.00

Is that the same as the Effective Diameter/ERD for spoke calculation purposes? 559mm in the above example? What's 21 in the above example? The rim width?

Thanks!

No ETRTO is the bead seat diameter. ERD is generally a bit smaller. Roger

So that's more like tire size then? Thanks.

Aha, DT calculator lists ERD for this particular rim (EX721) as 537mm.

The ERD is the diameter of the rim measured where the nipples are seated in the spoke holes, with the thickness of the two nipple heads added on. ERD is a key measurement in figuring spoke length for a wheel build. The bead seat diameter (559 for a 26" mountain bike rim, for example) is the diameter of where the tire's bead seats on the rim. The ETRTO system lists the bead seat diameter and width of the rim, for rims; bead seat diameter and width of the tire, for tires. The ETRTO system is for rim/tire sizing, ERD is a number used in spoke length calculations.

ETRO is actually the better way to talk about tire size. Most people would call that a 26" rim, but there are several variations that are all nominally called 26"; the rim size that is commonly referred to as 26" actually has an ETRO of 559mm, which specifies it exactly and causes much less confusion than calling it a 26" rim. Unfortunately, the ETRO standard has not caught on well, and the old confusing nominal terminology for tire and rim sizes still persists.