Abstract: Electrode assemblies are particularly used with a precordial overlay to obtain a quick and accurate positioning of electrodes for an electrocardiographic test. Each electrode assembly comprises an electrode to receive a lead wire, an underlying non-conductive pad secured to the electrode, and an overlying plastic sheet secured to the electrode in a manner whereby its peripheral areas are unattached. Preferably, the electrode assembly also has an underlying adhesive layer secured to the non-conductive pad and a removable backing strip. The precordial overlay has a set of slots in which the electrode assemblies are slidably mounted. The underlying non-conductive pad and the overlying plastic sheet of the assemblies effectively trap the electrode assemblies in the slots to allow sliding electrode movement. The sliding movement of the electrodes allows for limited but quick movement from an approximate chest area location to a precise chest area location for a reliable electrocardiographic test.