Genesis 38:15-18

"When Judah saw her, he thought her to be a harlot; for she had covered her face. And he turned unto her by the way, and said, Come, I pray thee, let me come in unto thee: for he knew not that she was his daughter-in-law. And she said, What wilt thou give me, that thou mayest come in unto me? And he said, I will send thee a kid of goats from the flock. And she said, Wilt thou give me a pledge, till thou send it? And he said, What pledge shall I give thee? And she said, Thy signet, and thy cord, and thy staff that is in thy hand. And he gave them to her, and came in unto her, and she conceived by him."

The stratagem worked. Tamar had completely outwitted him. Little could Judah have realized that he had just become the father of a great multitude through Tamar, including the Christ himself. Why did God permit such a thing? Simply because Tamar was a convert from the paganism to the true faith, and, by her, God would cut off the fountain head of paganism in the Chosen People, an influence which had already entrenched itself in the household of Judah through the Canaanite daughter of Shua.

It is useless to inquire about the exact nature of the signet, the cord, the staff, etc. They were simply valuable personal ornaments worn by Judah, and, what was most important to Tamar, they made in possible for her to identify the father of her child with absolute certainty.