A skinny/monkey
Although this type of boy looks slim, his body hidden under his clothes also has a little sexy muscles. At present, most of the popular herbivorous boys in Japan are of this figure.
But this type of perfect figure usually has three obvious characteristics.
Features 1: perfect clavicle.
Feature 2: Small waist without excess fat.
Feature 3: "obvious" biceps brachii.
So don't think that being thin belongs to this figure. You need to meet the above three points before you can sit in the right position.
Naturally slim/symmetrical
This kind of figure has no big muscles, but it doesn't look fragile, and it belongs to the typical figure of "dressing thin and undressing with meat".