12-lead ECGs are really only done with 10 physical leads. One lead is ground, six leads are each single-channel measures (V1 -V6) that wrap around the heart and the remaining three leads are used to create six channels - three single-channel measures, and the last three channels are calculated as diffs between them. V1-V6 have specific spots on the chest to live, but the remaining four leads (including ground) get one limb each, and generally work wherever you want on that limb.

(I used to be an EEG tech, and for us, the ECG was the primary thing we wanted to get rid of...)