Superscripts, Subscripts, And Level Indicators

1. Except as stated in b below, the subscript level indicator must be used to show that the symbols immediately following it appear on the first level below the baseline of writing. Subscripts may carry subscripts of their own. In such cases, the subscript level inidcator must be doubled, tripled, etc. to indicate subscripts on the second, third, or lower levels of writing.
   1. f_n
      

   2. 3_k
      

   3. a_(k+1)
      

   4. 
      

   5. 10_?
      

   6. 
      

   7. 10₈
      

   8. ?₃
      

   9. (CO)₂
      

   10. n_xy
       

   11. P_x+1y+1z+1
       

2. The subscript level indicator must not be used before a numeric subscript on the first level below the baseline of writing if the numeral is a right-subscript to a letter. The numeric subscript may contain a segmenting comma or a decimal point. The letter may carry one or more primes or a superscript, be taken from any alphabet in any type form, and may be part of a two-letter chemical abbreviation. However, the letter must not be used as a numeral in a nondecimal system or as part of a word or abbreviation.

   The subscript level indicator must be used with a numeral on the first level below the baseline if a subscript contains any symbol other than a numeral with its comma or decimal point, or if the subscript carries a superscript or subscript of its own. Subscripts on the second or lower levels of writing always require their appropriate subscript level indicators.

   1. X₁
      

   2. ax₂
      

   3. A₅₆
      

   4. X_1,ooo
      

   5. X_5.3
      

   6. X_.7
      

   7. 
      

   8. 
      

   9. i₁
      

   10. CO₃
       

   11. Na₃
       

   12. TE₁₂
       

   13. five₃
       

   14. X_2k
       

   15. Y_-2
       

   16. X_2+k
       

   17. a_m1
       

   18. X_3n
       

   19. X_y2