Have you ever wondered why you can go into Banana Rebuplic and you fit their size 4, but then go to, I don't know some other mall store, and you fit their size 2?
Well here is the reason:
Each clothing brand has its own "fit model" which is a certain size that they base all their patterns on. This fit model is usually based on their "target customer" (the ideal customer that they are trying to sell to) One brand might be designing for basketball players, or for older women, or for dancers, so within each of these groups there might be similiar body types, but between them there aren't many. Thus, there must be different sizing variations for each group or brand
And that's not all, sizing has also changed as time has. For example, you can buy a vintage dress or pattern and the size you might normally wear is usually smaller than the size on the vintage item or pattern.
While this may get annoying for the individual buyer, it is actually better for the whole industry of ready-to-wear clothing. It is a lot more difficult, however, to buy your typical size while shopping online. Which is why on a lot of vintage sites they will size their clothes according to actual measurements (i.e. 34" bust, 26" waist, 36" hips.) Some sites also provide a measuring chart which is the same thing, they just give the actual measurements a size (i.e. 2, 4, 6; sm, md, lg)
So, it is very important to know your measurements, and how to take them properly. Especially the three main ones: bust, waist, and hips. I will be posting a tutorial with photos on how to do so very soon.