ECG electrodes are disposable adhesive sensors that detect the heart's electrical activity for electrocardiography (ECG/EKG), transmitting signals via Ag/AgCl sensing elements through conductive gel (4-10% chloride salts in hydrogel/foam) to snap/clip leadwires. Types include foam-backed (breathable, long-term monitoring), non-woven fabric (water-resistant diagnostic), and thin-film (pediatric/sensitive skin); wet gels suit short ECGs, solid gels >24hr Holter. Standard 12-lead uses 10 electrodes: limb (RA right arm, LA left arm, RL right leg ground, LL left leg) form Einthoven triangle; precordial V1 (4th ICS right sternum), V2 (left sternum), V3 (mid V2-V4), V4 (5th ICS midclavicular), V5 (anterior axillary), V6 (midaxillary) precise placement prevents artifacts like wandering baseline.