The plus "10" rule is not so cut and dry. It is true that a 40 US, for example, is a 50 EU. The issue lies with the trousers. In a US sized suit a 40 usually comes coupled with a 34 pant waist. A 50 EU suit comes with a 50 pant waist. What does this "50" pant translate to? In pants the difference is "16" as the "10" is to jacket size. A 50 pant is a 34 waist and vice versa. So a 40 inch pant is a 56 EU pant.

This holds true for many European countries including Italy. Germany has a different formula for sizes in longs and shorts than the rest of Europe starting at 98 for 50L, 102 for 52L and so on. France and the UK are also slightly different, but closer in sizing.  They match their respective sizes to the standard European sizing discussed above.