The mishna on Shabbat 92a says that one is prohibited from carrying in his right or left hand, in his lap, or on his shoulder, because that's how the mishkan was carried, but he can carry in an unusual way such as in his mouth, in his shoe, between his belt and his body, and several others that are listed. The g'mara clarifies that we are talking about what is unusual in one's community; specifically it first says carrying on your head isn't ok because that's what the Huzalites do, and then says "we should pasken by the Huzalites? for them it's forbidden but not for us" (I'm paraphrasing). So from this g'mara it seems like carrying is defined by local conventions.
This answer and its comments on a question about chewing gum seem to say that even carrying in an unusual way is rabbinically forbidden, presmably for everybody. (That's kind of a tangent to that question, as the mouth would be the usual place for chewing gum so it's not unusual.) If I'm understanding that discussion correctly, it would be rabbinically forbidden for me to, say, carry my house key in my shoe (sans eiruv), and that's why people make special belt buckles and necklaces.
My question is about how we got from the g'mara to the present rule. On what is the prohibition based? I realize that the g'mara isn't the final word on halacha, but since it's part of our received tradition I assume this one was considered and rejected, so I'm curious about what took precedence.