The "21-year rule" is a legal principle that pertains to the duration of certain types of trusts, particularly in the context of estate and tax planning. According to this rule, a trust's duration is limited to a maximum of 21 years. This means that assets placed within a trust must be distributed or come to an end within 21 years from the date when the trust was established.

The 21-year rule aims to prevent the indefinite tying up of assets in trusts, which could potentially lead to the avoidance of certain taxes or other legal obligations. By imposing this time limit, authorities seek to ensure that trusts are not used solely as a means to avoid taxes or to maintain control over assets for an extended period.

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