The Boolean Expression is an expression that 
evaluates to be either true or false.

This second Boolean Expression is evaluated only if
the first Boolean Expression is false.

If this second Boolean Expression is true the code
associated with it will be executed. (STATEMENT B)  

If this second Boolean Expression is false, the code
associated with the else will be executed instead.
(STATEMENT C)


Example:


if (num == 0)                           if (BOOLEAN 1)
   printf ("num equals 0\n");              STATEMENT A 
else                                    else
   if (num < 0)                            if (BOOLEAN 2)
      printf ("num is negative\n");           STATEMENT B
   else                                    else
      printf ("num is positive\n");           STATEMENT C


In the example above, the second Boolean Expression is:

(num < 0)

If it is true, the code to execute is:

 printf ("num is negative\n"); 

If it is false, the code to execute is:

printf ("num is positive\n");


What the above code represents logically is a three-way 
decision, and to show this visually, the indentation is 
often arranged as follows:


if (num == 0)                          if (BOOLEAN 1)
   printf ("num equals 0\n");             STATEMENT A
else if (num < 0)                      else if (BOOLEAN 2)
   printf ("num is negative\n");          STATEMENT B
else                                   else
   printf ("num is positive\n");          STATEMENT C