Which size guide belongs to which brand
Short answer: Every brand has its own guide, and they are not the same. A medium at one brand can equal a small at another.
Why they differ
There is no shared standard for clothing sizes. Each brand builds its own scale from the body it designs for. That is why the letter alone carries no information.
How to find the right one
Look for the brand name on the product page and find the size guide belonging to that particular brand. The guide for another brand in the same shop does not apply. See JBS of Denmark.
When you buy several brands
Note your size per brand in one place. Then the next order takes thirty seconds. See Boody and Bamboo Basics.
Frequently asked questions
Can you trust the letter
Only within the same brand and the same range.
What if there is no guide
Then measure a garment you own and ask the shop for the product's measurements.
Do brands change their guide
It happens. Check the current one before ordering again after a couple of years.