A precordial overlay and dual backing assembly is expeditiously affixed together in a manner which allows it to be packaged in an unalterable form. When needed to obtain an ECG reading, the assembly is removed from the packaging and the precordial overlay swiftly separated from the dual backing. The dual backing is discarded. The precordial overlay is quickly and accurately positioned on a patient, The precordial overlay of the assembly comprises an elongated band having a first and a second generally rectangular-shaped sections joined together by an intermediate section. All three sections have at least one cut-out to receive an adhesive-backed electrode assembly. The dual backing of the assembly is similarly shaped and comprises a thin intermediate substrate with release agents on each side and a thin lower substrate with a release agent on a top side. The intermediate substrate further has a set of bent-back flaps, each flap positioned to receive an underside of one of the electrode assemblies. The flaps and the release agents of the dual backing allow the precordial overlay to be swiftly disassembled from the dual backing and used to obtain reliable test results in short order.