how come the capacity remains same after the third append?

gistfile1.txt

class Sb1{
public static void main(String[] args){
StringBuffer sb=new StringBuffer();
System.out.println(sb.capacity());
System.out.println(sb.length());
StringBuffer sb1=sb.append("abcdefghijklmnopqrstuvwxyzabcdefghijk");
System.out.println("after first append");
System.out.println(sb.capacity());

sb.append("q");
System.out.println("after second append");
System.out.println(sb.capacity());

sb.append("p");
System.out.println("after third append");
System.out.println(sb.capacity());
}
}