The LSB (least significant bit) is on the left and the MSB (most significant bit) is on the right. The top layer represents the bits of the first nybble and the bottom layer represents the bits of the second nybble.
The lower left bit is the Cin (carry-in) bit. The LED in the lower right corner is the Cout (carry-out) bit. This Cout bit will be activated when an overflow occurs where the two nybbles to be added exceeds 15.
