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The multiple ad units system serves ads to each ad unit in order of their appearance in your HTML code. In most cases, this will be the same order as the ad units appear on your page. However, if your code uses DIV tags or other positioning techniques, it's possible that our system will recognize an ad unit as the first ad unit, even if it appears to the end user at the very bottom of the page. Because our system may not always fill all of your ad units on any one page, it's important that you ensure that your 'main' ad unit appears first in the HTML code.
The above help seems insufficient to solve the problem.
I have a page on which I am using 728x90 and 468x60 banner. The 728x90 ads appear much before the 468x60 ads and its CTR is much lower than 468x60 size. I want to define 468x60 as my first ad unit. How can I do this using DIV code as stated on Adsense help pages?