When the GST was originally enacted, most state laws included the common law Rule against Perpetuities (RAP) or some similar rule.

FORBES: Obama Estate Tax Budget Proposals

It's probably hard to plan a year off if your wits have been dulled by the Uniform Commercial Code and the Rule Against Perpetuities.

WSJ: Elizabeth Wurtzel: Tough Times for Big Law

But since then, about half the states have repealed that rule or lengthened the "permissible perpetuities period, " in some cases to as long as 1, 000 years, Mr. Steiner says.

WSJ: Family Value: New Hazards for Estate Plans

To enable these trusts, most of the states allowing them had to get rid of an old common-law principle called the "rule against perpetuities, " which allowed trusts to exist only for about 90 years.

WSJ: Tax Report: Dynasty Trusts Under Attack