I just don't get it, or it's changed over the years.
The bands feel far too small for the sizing.
I used to buy a 34 band , B cup, (going back years) and then I did 'bra intervention' and realised I was really a 30 band,D cup. Sometimes C cup.
That worked for a while then the bands started feeling tighter (recent purchases.)
I have measured myself today. 29 inch under bust, 30 inches with a bit of 'give'.
But the M&S 32 bands I have are very uncomfortable. I measured them - they are 24 unstretched. That means they have to stretch a full 6- 8 inches to fasten.
On their website M&S say 'add 4 inches' to your under band measurement. We were all told years ago this was wrong- but is it something now that is unique to M&S to get the right size?