These conjunctions join clauses (subject + verb) or sentences to form a compound sentence, in which one clause contrasts or differs from the other.

Positive ( + ) Negative ( - )
	These schemes provide a solution, but they are not easy. (with comma) These schemes provide a solution but are not easy. (no comma) These schemes provide a solution, albeit a difficult one, to our current traffic problems.
Negative ( - ) Positive ( + )
	These schemes are not easy to implement, but they are nevertheless technically compatible.
Negative ( - ) Negative ( - )
	These schemes are not technically compatible, nor are they easy to implement. These schemes are neither technically compatible, nor are they easy to implement.
Positive ( + ) Positive ( + )
	These schemes provide a solution, and they are easy to implement. These schemes provide a solution and are easy to implement. (For the opposite strategy, see also and, also, both... and..., not only... but also...)