title: In Javascript, what does this underscore mean?
body: <pre><code>var Gallery = Backbone.Controller.extend({
      _index: null,
      _photos: null,
      _album :null,
      _subalbums:null,
      _subphotos:null,
      _data:null,
      _photosview:null,
      _currentsub:null,
      routes: {
          "": "index",
          "subalbum/:id": "subindex",
          "subalbum/:id/" : "directphoto",
          "subalbum/:id/:num" : "hashphoto"
      },
      initialize: function(options) {
          var ws = this;
          if (this._index === null){
              $.ajax({
                  url: 'data/album1.json',
                  dataType: 'json',
                  data: {},
                  success: function(data) {
                      ws._data = data;
                      ws._photos =
                      new PhotoCollection(data);
                      ws._index =
                      new IndexView({model: ws._photos});
                      Backbone.history.loadUrl();
                  }
              });
              return this;
          }
          return this;
      },
      //Handle rendering the initial view for the
      //application
      index: function() {
          this._index.render();
      },
  </code></pre>
  
  <p>I'm reading a tutorial on backbone.js here: <a href="http://addyosmani.com/blog/building-spas-jquerys-best-friends/" rel="noreferrer">http://addyosmani.com/blog/building-spas-jquerys-best-friends/</a></p>
  
  <p>What are the underscores? (_index, _photos, _album)  Why use them?</p>