What is the 'head of the corner' or chief stone in ancient architecture?
In ancient architecture, especially arches, the chief stone or head of the corner is the final stone placed at the center of the top of the arch. All the rest of the arch rests against that one, so it is the chief stone. Peter applies this to Christ, the Messiah Israel rejected.
Les explains that in the ancient architecture of arches, the final stone placed at the center of the top of the arch is the chief corner stone; all the rest of the arch rests against it. He also relates a legend, not Scripture, about Solomon's temple: every stone was pre-cut at the quarries so there was no hammer sound at the site. According to the legend, the chief stone came to the builders, but in confusion they could not find a place for it, so they dragged it to a vacant lot where weeds grew around it and people stumbled over it when taking shortcuts. Les says the story is apropos because Peter uses that analogy: Israel was the builder of God's Kingdom, and Christ was to be the Messiah, the chief corner stone holding it together; but Israel rejected Him and threw Him out, and He became a stone of stumbling to the nation ever since.