What is the correct height for OO gauge couplings?

The correct height and other dimensions for HO gauge couplings are covered by NEM document 360 which states that couplings should be positioned at a height of 8.5mm above rail height. OO gauge couplings appear to follow suit with most manufacturers making a reasonable effort to provide rolling stock with a coupling height of 8.5mm above the top of the rail. But what does all that mean in layman's terms?

Knowing which part to measure when presented with a figure can sometimes be a little mystifying, especially as a coupling has so many different parts. On tension lock couplings, the 8.5mm recommendation simply relates to the distance between the top of the rail and the top of the 'D' on which the coupling hook rests. This recommendation has a leeway of plus or minus 0.2mm, so doesn't have to be exact.

For NEM couplings, the 8.5mm measurement pertains to the top of the rail to the top of the pocket's hole into which you slot your actual coupling.

Of course, these recommendations are far from a guarantee that all rolling stock will have couplings fitted at this height, the age and brand of your models will both have a bearing on the coupling's resting position, but in general, a coupling height of 8.5mm is what you should be aiming for.