The restrictions imposed by the plain meaning rule on the introduction of extrinsic evidence of intent do not apply to inter vivos trusts: “If the meaning of the writing is uncertain or ambiguous, evidence of the circumstances is admissible to determine its interpretation.” Such evidence is permitted to aid in the construction of the language of a trust:
Oral evidence will be received, however, to remove an ambiguity in the construction of the trust instrument by explanation of the meaning of the words therein, based on the situation of the parties and other facts. This principle (applies) … both as to private and charitable trusts.
Indeed, in Maryland a trust of personalty may be created solely by parol evidence. Because parol evidence can be used to interpret trusts that were created inter vivos, parol evidence may also be used to reform or modify such a trust:
In trust law, a settlor’s unilateral mistake is sufficient to reform an inter vivos trust, provided the settlor received no consideration for the creation of the trust. The same rule applies even after the death of the settlor, provided the reformation is necessary to carry out his intent. Courts have frequently corrected scriveners’ errors by reforming unilateral mistakes in trust instruments. In addition, courts have corrected omissions resulting from scriveners’ mistakes. Because a revocable inter vivos trust can imitate a will, in that the settlor can retain the equitable life interest and the power to alter or revoke the beneficiary designation, the differing result hinges on terminology. Significantly, a scrivener’s error can serve as a basis to reform a pour over will. A court, however, generally will not reform a testamentary trust under similar circumstances, unless the will which contained the trust can be reformed. It seems arbitrary for the law to hold that an inter vivos trust used as a receptacle for assets poured over from probate can be reformed, while a testamentary trust cannot. If will substitutes, including revocable trusts, can be reformed for scriveners’ errors, then wills should also be able to be reformed under similar circumstances, especially when both kinds of instruments accomplish the same testamentary objectives.
A Maryland case held that after the death of the settlor, the beneficiary could press for a modification due to mistake to the same degree that the settlor could have brought such an action for modification.
 Restatement (Second) of Trusts § 38 (1959).