The most widely known rebel flag, also known as the Southern Cross. Based on the battle flag of Southern general P.T. Beauregard. This flag had a red field, with white-bordered blue diagonal stripes (called a Saint Andrew's cross). Within the blue stripes were the eleven white stars of the Confederate states, plus two for oppressed Kentucky and Missouri. It is properly a square flag, as were most battle flags: the wider 3x5 ratio version is actually the Confederate navy jack.
This is the flag that appears on the General Lee and many transport trucks from the South.