One may convey an estate absolutely or upon condition. A condition may be precedent, keeping an estate from vesting, or, it may be a condition subsequent which defeats a vested estate. A condition precedent is one which prevents any ownership accruing at all, as where an estate is to pass to one provided she marries. Thus, all contingent remainders are estates depending on conditions that may prevent them from ever vesting.
We shall notice an estate upon condition in the case of a mortgage which of old (and even now in form), was an estate subject to defeat by the payment of the mortgage debt, but by failure thereof, passing into an absolute estate.
So we shall notice other estates upon conditions as to the use of the land.