what is the historical meaning of the keystone in the royal arch and how do standard commentators interpret its symbolism

The keystone historically represents the central structural wedge or plug that binds a vaulted arch or dome together, providing the support and integrity required to complete the edifice. In operative architecture, this stone serves to lock the arch into place, a principle that was later interpreted in speculative ritual as an emblem connecting degrees and symbolizing structural stability, completion, and the concealment of sacred secrets, as a paper read before Quatuor Coronati Lodge notes.

Because the library contains no passages on this specific topic, the detailed interpretations of individual commentators cannot be provided here.

Nineteenth- and twentieth-century Royal Arch tracing boards depict the cardinal zodiacal sign of Cancer occupying the keystone of the arch 1. Jaime Paul Lamb notes that this emblem references the summer solstice within the tropical calendrical system, matching the Tropic of Cancer as shown on the terrestrial and celestial globes 1.

In an earlier precessional period—specifically during the epoch of Anno Lucis around 4000 BCE and throughout the Taurian age—the summer solstice occurred in the sign of Leo rather than Cancer 1. Placing Cancer on the keystone thus reflects a relatively modern astronomical orientation compared to the ancient period in which the ritual's historical setting is placed 1.

The library sources do not contain further interpretations or older readings of the keystone beyond this astronomical placement.