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Hello, the Great Wall of China is not a single wall but made of many smaller walls begun in the 6th century BC.

It's never been simply a national border and contrary to popular belief, it isn't visible from space.

Nevertheless, this astonishing fortification which snakes for thousands of miles through deserts and mountains to the sea has been a powerful symbol in Chinese life for millennia.

It was initially joined up at the command of the First Emperor in the wake of his creation of China in 2 to 1 BC.

But the wall which is most familiar to us today is larger the work of the Ming dynasty, 1368 to 1644.

Over the centuries the wall has come to symbolize strength and unity, failure and longevity.

It remains a potent image long after its military uses fell away.

With me to discuss the Great Wall of China, a Julia level, lecture in Chinese history, Birkbeck College University of London, Rhinemitter, professor of the history and politics of modern China at the University of Oxford and Francis Wood, head of the Chinese section at the British Library.

Francis Wood, if you travel at high speed down the length of the Great Wall today, what would you see?

You'd see an extraordinary variation from west to east. If you started in the west where the Great Wall tumbles down into the sea, from there moving westward slowly you would see a great fortification of grayish blue brick which it's between seven and nine meters high.

It's about five meters wide at the top so that five horsemen can ride along in parallel. It has watchtowers which stick up every couple of miles or so and it covers.

Really, it just snakes over the tops of the green mountains of North China. Sometimes you stand on the wall and you can see more walls further north.

You snake across west going westwards through the center of northern China. Sometimes there you'd find the wall made of what's called tiger skin, tiger skin brick which is when they use a kind of crazy paving of stone with white lime infill and above you get the gray brick crenellations.

Then you go further east and start getting towards the deserts of central Asia. The wall changes color considerably. Instead of being blue gray it becomes yellow and it's built there of tampt earth not brick mainly.

You can still see pieces of parts of the wall which were built over two thousand years ago just out of earth, out of pounded earth, yellow in color and they've been window-roaded wonderfully so that it's scooped out and hollowed like the inside of a shell.

So it's very different east or west. And what length are you talking about the total length now?

This is a wonderful question. People say I keep seeing the figure of 50,000 kilometers. People may disagree with that but that's a kind of standard figure that is trotted out but I think nobody actually knows.

50,000 kilometers. Well it's because you're talking about bits of wall here and bits of wall there. I'm sure that people will be able to be more precise later on.

Fundamentally you're traveling about sort of 6,000 miles from 6,000 kilometers sorry from east to west but you've got lots and lots of layer of wall but I don't think anyone knows.

Can you take us back six century lots of little walls, the warring states, building walls, earth walls often to keep the other warring states away from their particular warring states.

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