# What are the two measurements per box being done in the Bell Tests in the IBM Quantum Experience "Entanglement and Bell tests" Section

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.

Both $$A$$ and $$A'$$ measure the same qubit, while $$B$$ and $$B'$$ are applied to the second qubit. Alice chooses (randomly) which of the two measurement settings ($$A$$ or $$A'$$) she will choose for each run of the experiment, while Bob chooses between $$B$$ and $$B'$$.