It began when I started drawing Undertale, and it's entirely Papyrus' fault, lol. I had a lot of trouble drawing Papyrus in my style in a way I liked until I gave him the gap. I thought it looked cute and suited him. Then I thought I'd give Sans a gap too, inspired by other artists who draw him that way, and also cause I thought it'd make sense as he and Papyrus are brothers - like, they look so different from each other, but they have a similar trait that links them as related, something like that. With that logic, when I started drawing Handplates!Gaster, who is the clonedaddy of the brothers, it made sense to give HIM a gap as well. Same idea with Subject 3. When it comes to the skeletons, I try to use their teeth and the way they're shaped to show their different personalities.