D who typically wears a size 8 would wear the 38 and could fit a thinner sock with them - if you want to wear a thick sock i'd recommend sizing up. and L who usually wears a US 8.5 but has a wider foot would take the 39. We consider this true to size.
Original BIRKENSTOCK footbed: anatomically shaped cork-latex footbed
Details: strap with two individually adjustable metal pin buckles