Quotes About Keystones

I found that through my life, living in the city of Toronto, I look above the Pizza Pizza sign, and I look above the other signs and window dressing, and I see evidence of a city that no longer exists in the keystones and the decorations that line the tops of buildings. That presence of the old city has always moved me.

What are the guidelines for choosing the size and strength of the chain links utilized to retain 'keystones'?