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I am confused as to what is being measured in the boxes in the example drawings shown on the Entanglement and Bell Tests section in the IBM Q Experience:

https://quantum-computing.ibm.com/docs/guide/mult-entang/entanglement-and-bell-tests

My understanding is that the $(|00 \rangle + |11 \rangle)/ \sqrt{2}$ represents a superposition of two states, $|00 \rangle$ and $|11 \rangle$. And that $|00 \rangle$ is a two qubit entangled state. If Alice measures one of the qubits to be a $0$, the other qubit must be a $0$ as well due to the entanglement. The same is true if Alice measures a $1$.

My question comes from not understanding why $A$ and $A'$ are present in the box. What are they measuring? Is $A$ measuring one qubit while $A'$ is measuring the second qubit? Or is $A$ measuring one qubit while $A'$ measures the same qubit. Same questions for box $B$.

MY BACKGROUND: I am working through the IBM Q Experience. I have some exposure to QM through College level Physical Chemistry. I have a basic understanding of linear algebra, superposition, and entanglement.

In the first formulation of a Bell Game I saw, it was presented with each player either moving a joystick attached to their box either to the left or to the right.

Here, is Alice picking between A and A', while Bob is picking between B and B'? – Abstract Acumen – 2020-05-14T19:28:44.573

1Yes, that's correct. – M. Stern – 2020-05-15T16:59:54.073