Dimensions of Outer Corner Mouldings

[MagHam]

Sitting here planning some "fine carpentry" with boards and exterior moldings...

Neither XL, Beijer, ByggMax, nor K-rauta specify all sub-dimensions of exterior corner moldings online as far as I can see.
Just that they are available in e.g. 27x27 or 33x33

Does that refer to dimension "a" or "b" in my image? If so, what are the standard measurements for the others?
(if you know two, you can figure out the third, of course)

The last resort is to fit in a weekend visit to the hardware store and measure on-site... :|

 

[corre]

Based on Södra, I say outer dimensions. http://www2.sodra.com/sv/Sodra-Interior-Sverige/List/Lister/Hornlist/Hornlist-i-furu-27x27-mm/

 

[MagHam]

Ahh! 27 but "covers 21". That was the answer to the question.