Note to Hamlet, 2.2.342: "the common stages"
"Common stages" refers to the large outdoor "public" theaters, such as Shakespeare's Globe. The boys played at the royal court and in the indoor Blackfriars Theatre, which was referred to as a "private" theatre, even though it was open to anyone who had the money, which was four times the price of admittance at the much larger "public" theaters.
Conjectural Reconstruction of the Interior of the Blackfriars Theatre
Conjectural Reconstruction of the Interior of the Globe Theatre