These two samples are regarded as Flat Base crowns:

These two samples below are regarded as Curved Base crowns.
I think the "base" would be the bottom, but I don't understand why reference books call one flat and the other curved. They look elliptical or maybe "more oval" and "less oval". Your thoughts?


These two samples below are regarded as Curved Base crowns.
I think the "base" would be the bottom, but I don't understand why reference books call one flat and the other curved. They look elliptical or maybe "more oval" and "less oval". Your thoughts?





















