The lapel is the fabric that runs from the button, around the neck, back down to the button. Not only are there the peak, notch, and shawl but there is 2" all the way to 4.5" if you would like. How do you know what to pick for your body type? Let's talk about five of your options.
I'll mention as well, when we are talking about the width, it is at the widest point where the notch or peak in the lapel comes out.
The golden rule, the width of your tie and the width of your dress shirt should match the width of your lapel. A wide 4.5" peak lapel should not be worn with a 2" slim tie. It's all about proportions.
This is meant to be a quick rundown so if you have any questions please comment below.