# Why φ?

The most common alternative in use at the time was the use of Fraktur letters, e.g., $$\mathfrak{A}$$ as a metavariable for formulas, and A as a formula variable; x as a bound variable and $$\mathfrak{x}$$ as a metavariable for bound variables. This was the convention in the Hilbert school, also followed by Carnap. Kleene later used script letters for metavariables and upright roman type for the corresponding symbols of the object language. But indicating the difference by different fonts is perhaps not ideal, and Fraktur may not have been the most appealing choice anyway, both because it was the 1940s and because the type was probably not available in American print shops.