A more thorough explanation:

The doctrine of integration of wills is a legal theory that explains how multi-page wills are treated as a single document. According to this doctrine, a will does not have to be signed or initialed on every page as long as all pages are present at the same time and the testator intended all of the separate pages to be part of the same will.

For example, in the case of Estate of Thomas J. Twohig, the court held that two or more handwritten documents that do not refer to one another may be admitted to probate when it is clear that the testator intended them to be his will. This means that even if the pages of a will are not physically attached to each other, they can still be considered part of the same document if the testator intended them to be.

It is important to note that the doctrine of integration is different from the doctrine of incorporation by reference. The latter concerns situations where the contents of a separate writing are given effect as terms of the will, even though the separate writing itself is not considered part of the papers constituting the will.

In summary, the doctrine of integration of wills allows multi-page wills to be treated as a single document as long as the testator intended all pages to be part of the same will and all pages are present at the same time.