An electrical connector includes a main board (5) with a first card edge connector (1) mounted thereon, a sub-board (6) with a second card edge connector (2) mounted thereon, and a pair of complementary board to board connectors (3, 4) mounted on the main board (5) and the sub-board (6) respectively. The first and second card edge connectors (1, 2) are aligned with each other in a vertical direction and the second card edge connector (2) is on top of the first card edge connector (1).