The liability of a subscriber to corporate stock exists by virtue of the contractual obligation to the corporation to pay the subscription price or the unpaid installment thereon. Because this liability is often declared by statute, it is essential, to avoid a confused analysis of the precise nature of the liability in question, to distinguish other types of stockholder's liability. Statutory super-added liability in excess of the par value of the stock, and liability for watered stock are excluded from consideration. An analysis of the subscriber's liability will be materially aided by a classification with respect to plaintiffs entitled to compel the payment of the unpaid amount.