How to represent multiple elements of the same set correctly?

In a mathematical model, I have a set $$J$$ of distinct jobs, and a constraint dealing with pairs of elements from the job set. Which of the following is the most accurate way to address two jobs $$j$$ and $$j'$$ from the set $$J$$?

1. $$j,j' \in J$$

2. $$(j,j') \in J$$

3. $$j,j' \in J^2$$

4. $$(j,j') \in J^2$$

In short, I want to know whether I need brackets, and the set should be $$J$$, or $$J^2$$?

** Perhaps a basic question, but I want to be accurate in my writing.

• From the perspective of a guy who have little experience in math, 1. is the easiest to understand (and 5. of Rob Pratt answer if you want to be more correct). – mrBen Mar 4 '20 at 12:22

1. $$j\in J,\ j'\in J$$
2. $$(j,j')\in J\times J$$