It should be placed under the breast.
The electrode should be placed under the breast.
An interesting question. As you may know way back when the original practice was to attach EKG leads to the wrists and ankles. Modesty was an issue. You have two problems. Degradation of signal and artifact. With a 12 lead there will be some diminishment of signal. It is a better idea to place the electrodes under the breasts, even though that involves a bit of privacy invasion. This is ocasionally a problem with paramedics, who don't use a 12 lead. An ST depression will usually show up in either case.