Thread: about typename keyword

I am reading thinking in c++ book(Vol.2).I am in template section amd i don't understand typename keyword's meaning and usage.

Here is sample code:

Code:

#include <iostream>
#include <list>
#include <memory>
#include <vector>
using namespace std;
 
template<class T, template<class U, class = allocator<U> >
         class Seq>
void printSeq(Seq<T>& seq) {
  for(typename Seq<T>::iterator b = seq.begin();
       b != seq.end();)
    cout << *b++ << endl;
}
 
int main() {
  // Process a vector
  vector<int> v;
  v.push_back(1);
  v.push_back(2);
  printSeq(v);
  // Process a list
  list<int> lst;
  lst.push_back(3);
  lst.push_back(4);
  printSeq(lst);
} ///:~

Here why do we write "typename Seq<T>::iterator b" .Maybe i don't understand because Seq<T> is special library.Is it a type of library?

I really don not understand typename keyword.

Can someone explain me that please?
Thanks.