Hatching is a great way to start introducing some shading and texture into a form in the ink drawing. They're basically lines working in exactly the same way layered beside each other - the nearer together the strains, the darker the shadow. These traces will vary hugely in size https://howtodrawwithinkpens25803.mdkblog.com/42010306/a-simple-key-for-ink-pen-drawing-techniques-unveiled