This section is from the book "The Law Of Real Property and Other Interests In Land", by Herbert Thorn Dike Tiffany. Also available from Amazon: A Treatise on the Modern Law of Real Property and Other Interests in Land .
The language, in a deed or other instrument conveying or creating an estate, which by express language or by implication indicates the quantum or duration of the estate created, is termed the "limitation of the estate," as fixing its limit. Accordingly words which serve to indicate the duration of the estate are called "words of limitation," and are to be distinguished from "words of purchase," which state the person or persons intended to take the estate or estates limited. A number of words, such as "heirs," "issue," "children," etc., are capable of use either as words of limitation or as words of purchase, and the determination of the purpose of their use in a particular instrument is frequently a matter of difficulty.7
5. Leake, Property in Land 7. See Digby, Hist. Real Prop. 43.
6. Williams, Real Property 434.