May 25, 2019 at 11:29 am #33620
We answered this directly but it is good to share here, others may have the same question.
See GTM 11.3. It is determined by whether or not grade one mode is already in effect. It specifies the prime sign in this rule. Don't confuse the rules for the double g (used only in the middle of a word) with the rules for the prime (even though the braille symbol is the same. See also the example on page 49 of GTM where it occurs with the cup and cap (grade one passage is used there)
Take the symbol in context, which is math.