An estate upon condition subsequent is one which will be terminated by the happening or not happening of a certain event. The condition may be either one involving the use of the land or it may be a collateral one. The happening of the event also may depend upon the act or will of the holder of the conditional estate or may be independent of his control. For example, the condition might be that a certain part of the property should not be built upon; that the holder of the estate could not remove from a certain place; or that a certain event entirely beyond the control of the holder of the estate should not take place.

If the condition is illegal or against public policy, the condition is disregarded and the estate becomes an absolute one. A striking contrast is to be observed in this respect between the effect of illegality on a contract of this kind and the effect of illegality upon a condition precedent.

Upon the breach of a condition subsequent there must be an entry by the person entitled to the reversionary estate in order to work forfeiture. There can never be a remainder after an estate upon condition subsequent.2

The law at the present time looks with very great

2 " The second thing is, that no entry nor re-entry (which is all one) may be reserved or given to any person, but only to the feoffor, or to the donor, or to the lessor, or to their heirs, and such re-entry cannot be given to any other person. For if a man letteth land to another for term of life by indenture, rendering to the lessor and to his heirs a certain rent, and for default of payment a re-entry, etc., if afterward the lessor by a deed granteth the reversion of the land to another in fee, and the tenant for term of life attorn, etc., if the rent be after behind, the grantee of the reversion may distrain for the rent, because that the rent is incident to the reversion; but he may not enter into the land and oust the tenant, as the lessor might have done, or his heirs if the reversion had been continued in them, etc. And in this case the entry is taken away forever; for the grantee of the reversion cannot enter, disfavor upon conditions subsequent (which have the effect of unsettling land titles) and the tendency at the present time is to substitute restrictions for those conditions affecting the use of the land, and to do away with collateral conditions subsequent.