Understanding the $ in Java's format strings

 StringBuilder sb = new StringBuilder();
 // Send all output to the Appendable object sb
 Formatter formatter = new Formatter(sb, Locale.US);

 // Explicit argument indices may be used to re-order output.
 formatter.format("%4$2s %3$2s %2$2s %1$2s", "a", "b", "c", "d")
 // -> " d  c  b  a"

In this case, why is a 2 appended to $?

The 2 has nothing to do with the $:

• %     =   Start of format string
• 4$   =   Fourth argument ('d')
• 2     =   width of two (right-aligned)
• s     =   type of String