From the standpoint of limitation of the indebtedness under the mortgage, there are three generally accepted classes. The first is known as a "closed mortgage" under which no more indebtedness can be incurred. The second is an "open mortgage" under which the indebtedness may be increased; in other words, the amount authorized under the terms of the mortgage has not been reached - the opposite of a "closed mortgage." The third is the "open end mortgage" where there is no fixed limit placed upon the indebtedness; under Certain conditions, imposed by the deed of trust, it may be increased at will. To illustrate: The Baltimore & Ohio Railroad has a general mortgage, which, when created, was limited to $600,000,000, but it was provided that the stockholders could, by vote, increase the amount. It is also provided that at no time shall the outstanding amount, plus all "underlying bonds," exceed three times the capital stock. It will thus be seen that there are certain conditions restricting further issues under such a mortgage as this.