Why is it called 12 lead ecg if it only uses 10? pls explain

The EKG is routinely: performed with 6 chest wires (V1-V6) (six signals or leads) and the four arm wires. 10 wires. The signals from the arm and leg wires are electronically combined to provide a triangular pattern of three leads (signals) I,II,III. And then also electronically combined to produce what is called augmented leads (signals). aVR, aVL, and aVF. 6 more signals (leads). Thus 10 wires and twelve leads.