The Black-capped Chickadee is very similar to the Carolina Chickadee and where their ranges overlap they can be difficult to separate. In fact, they have trouble telling themselves apart and hybrids occur. The most obvious difference between them is their songs. Black-capped sings a two note song while Carolina sings a four note song. Hybrids sing a three note song. Outsided of the breeding season, when chickadees don't sing much, the Black-capped is slightly larger, has more white edging in the wing and has slower, lower-pitched calls.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.