Ok, I have to admit I'm challenged when it comes to getting things centered. I love this tool!!!!! It's the best thing around! One of the features I like about this type of ruler is that it takes the guess (calculation) work out of centering things that are a little off. It's easy to measure the center of a 6" peice of paper but when that piece of paper measures 5 1/3 " this makes it a little easier and much less time consuming.
I would recommend this ruler to anyone 100%. The two best things about it, is first, it is clear so you can see right where you are putting it. The second is, it is 18 inches long and so it is easy to work with 12x12 cardstock or paper. It also has the line break down one center side of the ruler and small dots down the other center side, which makes it easy to line up dots or a center line very easily.
I bought 4 12 inch and 2 18 inch of these rulers and I love them. They are so flexible but do not break. They measure accurately and allow the pefect centering of documents or cards.