D-shape compared to U-shape neck

I have a potentially dumb question...

How does a D-shape neck compare to a U-shape?

If I had to guess I would say D-shape necks have a bit less shoulder (maybe between U and C?) and are typically slimmer.

I think your observations are correct. Really, there isn't much difference between a U and a D other than how fat the neck is assuming all else is the same (nut width and radius). I wish all my necks were either a U or a D profile!!

Years ago (maybe beginning in the '60's??), the classification of Fender necks went A,B,C and D. The A was the very narrow....think Jazz bass. I have seen a few of these on guitars..especially from the mid-late '60's when narrow was the rage. The B is what one usually saw on Teles and Strats...round with no shoulders. C was round with some small shoulders and the D had big shoulders.
I would equate the U with the old D.