The stone that the builders rejected has become the cornerstone.
The habitual reading of this, at least in every homily I've heard, seems to veer wide of the sense, but in an interesting way. The original is "head of the corner", or the topmost piece at the angle, so it's unsurprising that a stone that wasn't used in the earlier construction is used for this late, critical purpose. (And it would be rather difficult to add a cornerstone at the end.) The usual reading, though, can be reduced to Homer Simpson shaking his head and muttering "Stupid builders." And there's a genealogy to that thought.
But the point is that the stone that wasn't built into the structure of things connects the angles at the top, uniting the two directions, and inner and outer. The Vulgate's lapis angularis captures some of this, perhaps suggesting the keystone of an arch, which plays a function in a vertical plane that the head of the corner plays in a horizontal plane. The Old English (Thanks, Alfred) version of wealstone, the big flat thing that provides most of the wall, would appear to have taken the error and magnified it.
The builders are building something. It's not necessarily a good thing. They use strong, flat rocks for the base, and as it ascends, rocks of lesser structural integrity can be worked into the mix. But then they reach a point that has little to do with sustaining vertical pressure. Another dimension of necessary integrity has arisen. And there's this rock off to the side that they hadn't wanted to build into the mass of the structure.
#notexpert #justwiseacring