Common atrium (CA) is one atrium but there are two normally situated atrial appendages. Right side of the CA resembles right atrium (RA) with superior and inferior vena caval connections. Left side of CA resembles left atrium (LA) with pulmonary veins communications. Infants are cyanotic due to admixture of deoxygenated blood in the CA. Symptoms of heart failure may be present. Ellis-Van-Creveld syndrome is often associated. Uncorrected defects develop pulmonary vascular disease. Common atrium is diagnosed when cyanosis is present in setting of large atrial septal defect (ASD). ECG findings are typical of ostium primum ASD like RBBB and left axis deviation. Absence of atrial septum with two AV valves and intact interventricular septum are diagnostic feature of echocardiogram. Angiogram shows globular single atrial chamber. Angiographic sign is very typical. Surgical repair is the definite treatment.