Where Should Using Statements Be Located

Question

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What is the difference between these two declarations?

I recently started working on a project with using statement located inside the NameSpace block.

namespace MyApp.Web
{
  using System;
  using System.Web.Security;
  using System.Web;

  public class MyClass
  {

I usually put my using statements above the namespace block.

using System;
using System.Web.Security;
using System.Web;
namespace MyApp.Web
{
  public class MyClass
  {

I don't think it matters, but I am currious if someone else had a recommendation and could they explain why one way is better than another.

Note: I always have one class per file.