Excel interop: _Worksheet or Worksheet?

I'm currently writing about dynamic typing, and I'm giving an example of Excel interop. I've hardly done any Office interop before, and it shows. The MSDN Office Interop tutorial for C# 4 uses the _Worksheet interface, but there's also a Worksheet interface. I've no idea what the difference is.

In my absurdly simple demo app (shown below) either works fine - but if best practice dictates one or the other, I'd rather use it appropriately.

using System;
using System.Linq;
using Excel = Microsoft.Office.Interop.Excel;

class DynamicExcel
{
    static void Main()
    {
        var app = new Excel.Application { Visible = true };
        app.Workbooks.Add();

        // Can use Excel._Worksheet instead here. Which is better?
        Excel.Worksheet workSheet = app.ActiveSheet;

        Excel.Range start = workSheet.Cells[1, 1];
        Excel.Range end = workSheet.Cells[1, 20];
        workSheet.get_Range(start, end).Value2 = Enumerable.Range(1, 20)
                                                           .ToArray();
    }
}

I'm trying to avoid doing a full deep-dive into COM or Office interoperability, just highlighting the new features of C# 4 - but I don't want to do anything really, really dumb.

(There may be something really, really dumb in the code above as well, in which case please let me know. Using separate start/end cells instead of just "A1:T1" is deliberate - it's easier to see that it's genuinely a range of 20 cells. Anything else is probably accidental.)

So, should I use _Worksheet or Worksheet, and why?