What are some slave borders in the US?

Answer 1

Should this question be referring to the United States before slavery was abolished in 1865, here are a few slave holding States that bordered on States were slavery was illegal:

Slave State Maryland:

borders with non slave States of: Pennsylvania and West Virginia;

Slave State of Delaware bordering non slave States of New Jersey and Pennsylvania.

This is only a short list of slave States bordering non slave States at the time of the US Civil War.