An example of the doctrine of merger in successive estates would be a case where a widow is given a life estate and a child (from a previous marriage) is given a remainder interest in a home or farm. The child purchases the life estate from the widow, thus owning both the life estate and the vested remainder interest. After the sale, the child owns the home or farm in fee simple absolute. (I've never seen this actually happen.)
Merger applies any time that the owners of successive estate are the same, even if there are other owners before them or after them in succession.
The doctrine of merger also applies when the same person owns a mortgage or deed of trust and the encumbered property, for example, when a secured lender buys property from the owner in a deed in lieu. Unless a contrary intent is manifested in the record, the mortgage or deed of trust and the ownership interest merge. But, this can be problematic if there is more than one mortgage on the property and, for example, the underlying ownership of the property is purchased by a second mortgage holder who would lose a right of redemption in event of a foreclosure if the interests merge. (This is a quite common fact pattern.)
The doctrine of merger also applies to contiguous subdivided properties. Suppose that A owns Blackacre and B owns Redacre, which are adjacent 15 acres parcels in a state where local government approval is required to carry out a subdivision when any of the resulting parcels has less than 35 acres. If A buys Redacre from B, then Blackacre and Redacre automatically merge into a single parcel of property and local government approval must be obtained to subdivide the property if A then wants to sell Redacre to C.
This kind of merger can also be relevant to cases of adverse possession. Suppose that there is a 15 year old fence between Blackacre and Redacre that is two feet past the property line into Blackacre, in a state where the adverse possession time period is 18 years. If A who owns Blackacre buys Redacre from B and then sells it to C, a month later (after obtaining local government subdivision approval), C's adverse possession period for the fence that is in a different location from the property line restarts when C takes possession, rather than tacking with the adverse possession period of B that would have taken place if C had purchased Redacre directly from B, because the property line ceased to exist when A purchased Redacre and Blackacre and Redacre merged into a single parcel. (There is a case in Colorado with this fact pattern.)
Another fairly common issue is when mineral rights in property are owned by the same person as the surface rights and then the same person owns both, which results in a merger in the absence of contrary intent in the record. So, while a deed of property in which the seller only owns the surface rights that does not mention mineral interests is conveyed to a third party, that person takes only the surface rights because you can't take more than the seller owned. But, if the seller had purchased the mineral rights before conveying to a third party in a deed that doesn't mention mineral rights, then the buyer would take both the surface rights and the mineral rights, because the surface estate and mineral estate would merge by operation of law.
A may avoid the merger by forming a company or trust to purchase Redacre, rather than purchasing it in A's own name. A purchase through a company or trust can also be used to avoid a merger of a mortgage and an ownership interest, or a life estate and a vested remainder interest, or mineral interests and surface rights.