Browsing MathOverflow I came across a question about analytical sets. Through the discussion following a comment made by our very own Asaf, I learned that bold face $\mathbf{\Sigma^1_1}$ and light face $\Sigma^1_n$ are different things.
Now, I am not asking about the definition or the distinction (I can look them up myself in a set theory book if I have time). I am asking about the history of this, in my not-so-humble opinion, ill-advised notation.
Does anyone know who is responsible for (or the history behind the development of) the convention that the same symbol in bold face means the projective hierarchy and in light face means the analytical hierarchy?