What creates a vacancy. Montana's enactment lists seven events: a designated trustee rejects the trusteeship; a designated trustee cannot be identified or does not exist; a trustee resigns; a trustee is disqualified or removed; a trustee dies; a guardian or conservator is appointed for an individual serving as trustee; or a trustee is ordered committed. Two of those are worth pausing on, because they are the ones families do not anticipate. A named person who simply never takes the job on creates a vacancy by inaction. And the appointment of a conservator over a trustee's own affairs ends their service, which is precisely the scenario in which an aging settlor-trustee is most likely to need a successor.
Whether it has to be filled, and by whom. If one or more cotrustees remain in office, the vacancy "need not be filled". It "must be filled if the trust has no remaining trustee." Where it must be filled, the order of priority is fixed: first the person designated in the terms of the trust to act as successor, then a person appointed by unanimous agreement of the qualified beneficiaries, then a person appointed by the court. A well-drafted document therefore never reaches step two, and the practical value of naming a second and third successor is that it keeps the family out of a negotiation requiring unanimity, and out of a courtroom.
Proving authority is the part nobody warns people about, and the statute has a clean answer. Instead of furnishing a copy of the trust instrument to anyone who is not a beneficiary, the trustee may furnish a certification of trust. It states that the trust exists and the date the instrument was executed, the identity of the settlor, the identity and address of the currently acting trustee, the relevant powers of the trustee, whether the trust is revocable and who holds any power to revoke, the authority of cotrustees to sign and whether all of them must, and, if another state's law governs, which state's. It must state that the trust has not been revoked, modified or amended in any way that would make those representations incorrect, and it "need not contain the dispositive terms of a trust." In other words, the bank learns who can sign and does not learn who inherits.
The protections around it run in both directions. A recipient may require the trustee to furnish copies of the excerpts from the instrument and later amendments "that designate the trustee and confer upon the trustee the power to act in the pending transaction", and no more. A person who acts in reliance on a certification without knowing its representations are incorrect is not liable for so acting, and may assume the stated facts without inquiry; a person who in good faith enters into a transaction in reliance on it may enforce that transaction against the trust property as if the representations were correct. And the sharp end: "a person making a demand for the trust instrument in addition to a certification of trust or excerpts is liable for damages if the court determines that the person did not act in good faith in demanding the trust instrument."
The first duties on taking over. Acceptance triggers the notice clock: within 60 days of accepting the trusteeship the new trustee must notify the qualified beneficiaries of the acceptance and of their own name, address and telephone number. A second 60-day notice is owed on learning that a trust has become irrevocable, "whether by the death of the settlor or otherwise", telling the qualified beneficiaries of the trust's existence, who the settlor was, and their rights to request portions of the instrument and to a trustee's report. Both are subject to whatever the instrument specifically limits or waives. And the exit works symmetrically: on a vacancy, unless a cotrustee remains in office, a report must be sent to the qualified beneficiaries by the former trustee — or, where that trustee has died or lost capacity, by their personal representative, conservator or guardian on their behalf.
What the role is not. A successor trustee's authority runs to the trust and to nothing else. It does not cover property the settlor never retitled into the trust, it does not cover medical decisions, and it does not survive as authority over an individual retirement account, which cannot be owned by a trust at all. Those gaps are why a plan pairs a trust with a durable power of attorney, a healthcare directive, and current beneficiary designations, and why a successor trustee frequently discovers on day one that the most urgent asset is one the trust does not hold.