How to create a tileable line pattern?

Hello,

How can I create a tileable line pattern like the one on the image? (Illustrator CS5)

Is there a more practical way than drawing and duplicating lines? When I do that, and fill a shape with the pattern, I see gaps between tiles. Do you remember the same pattern on Microsoft Word? I want to create that, on Illustrator. Which method should I follow?

Thanks.