The "catenary arch" is an arch profile which is essentially the inverted profile of a suspended chain. When you take a length of chain and suspend it upside down, it forms a natural "U," If you flip it over, it reflects a profile that's perfectly balanced.
If you're building an earth oven, this is a really cool way to assist you in building a sand form that's not lopsided.
The oven I'm in the process of building will be 15" high with a floor diameter of 31-1/2".
To make the arch, you need a length of chain and a piece of masonite or thin plywood.
On the plywood make a mark a zero inches and 31-1/2 inches. Find the center and make a mark 15 inches down. Hang the chain as shown, adjusting it until you get it to line up with your measurements, and with a felt tip, trace the chain.
You'll need a reciprocating saw to saw out your pattern