The Core Problem: There is No Universal Standard.
Unlike shoe sizes, which have a more direct relationship to foot length, there is no single, global standard for clothing sizes. A "Medium" (M) or a "Size 12" can vary dramatically between countries, brands, and even different styles from the same brand.
This is due to several factors:
Target Market: A brand's "fit model" (the person they design the garment on) reflects their ideal customer's body type.
Vanity Sizing: Many brands, especially in the US and UK, use "vanity sizing," where they label clothing with a smaller size number than the actual measurements to flatter the customer (e.g., a garment that measures like a 14 might be labeled a 12).
Style and "Ease": A "slim fit" shirt and a "relaxed fit" shirt in the same size from the same brand will have very different measurements. The amount of extra room built into a garment is called "ease."

A: Never Shop by the Label Alone: Your size is not a fixed number; it's a range that changes with every brand.
B: Read Reviews: Look for phrases like "runs large," "true to size," or "size down." Customers often share their own measurements and the size they bought.
C: Know the Fabric: Stretchy materials (jersey, elastane blends) are more forgiving than rigid ones (denim, 100% cotton).
D: Understand the Fit: Know the difference between "slim," "regular," "relaxed," and "oversized" fits.
E: When in Doubt, Size Up: It's generally easier for a tailor to take a garment in than to let it out.