Line of Scrimmage Rules?

[Official]

lets not confuse ourselfs. I've already learned a lot here in my short time. I'm understanding the well-explained answers. May be my question wasn't clear enough. I understand eligibilty of receivers. I'm wanting to know if you have to have a body part physically touching the line to be considered "on the line". I'm borrowing a rule book by the way and will read it cover to cover and ask more questions after that. I've got out on the playing field a couple times and reffed and learned loads out there. I'm just soaking up knowledge anyway I can in the mean time.