Exodus 26:4-5

And thou shalt make loops of blue upon the edge of the one curtain from the selvedge in the coupling; and likewise shalt thou make in the uttermost edge of {another} curtain, in the coupling of the second. {selvedge: an edge of cloth so woven that it cannot unravel} Fifty loops shalt thou make in the one curtain, and fifty loops shalt thou make in the edge of the curtain that {is} in the coupling of the second; that the loops may take hold one of another.