Exodus 26:4-5

4And you shall make loops of blue yarn on the edge of the curtain on the selvedge of one set, and likewise you shall do on the outer edge of the other curtain of the second set.

5Fifty loops you shall make in the one curtain, and fifty loops you shall make on the edge of the curtain that is on the end of the second set, that the loops may be clasped to one another.