What does "%%%ds" mean in a c program formatting text?

This c code I wrote below to centre text in strings works. I cannot find an explanation of what "%%%ds" in the program means and how it works. There is d for int and s for string but the three percentage symbols is obscure to me. What does "%%%ds" mean and how does it work?

/************************************************
formattCentre.c
example of formatting text to centre
************************************************/
#include<stdio.h>
#include<string.h>

char *verse[] =
{
  "The quick brown fox",
  "Jumps over the lazy dog",
  NULL
  };

int main()
{
        char **ch_pp;

 /* centre the data */
   
        for ( ch_pp = verse; *ch_pp; ch_pp++ )
        {
            int length;
            char format[10];
            length = 40 + strlen ( *ch_pp ) / 2;  /* calculate the field length  */
            sprintf ( format, "%%%ds\n", length ); /* make a format string.       */
            printf ( format, *ch_pp );       /* print the lines*/
        }
      printf( "\n" );
}

After this call

sprintf ( format, "%%%ds\n", length );

the string format will look like

"%Ns\n"

where N is the value of the expression length.

The two adjacent symbols %% are written in the string format as one symbol %.

From the C Standard (7.21.6.1 The fprintf function)

8 The conversion specifiers and their meanings are:

% A % character is written. No argument is converted. The complete conversion specification shall be %%

Try this simple demonstration program.

#include <stdio.h>

int main( void )
{
    char s[2];

    sprintf( s, "%%" );

    puts( s );
}

Its output is

%

So the next call of printf will look like

printf ( "%Ns\n", *ch_pp );