Define the 'first' ad unit when using multiple units
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.
Make sure that the best performing ad unit (highest CTR) is the first in your HTML code to improve your revenue.
Keep your ads looking like ads
Choosing ad colors that complement your site is great, but avoid aligning images with your ads or making nearby content mimic their formatting. Displaying ads in these ways isn't permitted by our program policies.
Call them what they are
While it may be tempting to call them "resources" or to place them immediately below your site's headings, take care to avoid labels and headings that may mislead users. Ad units may only be labeled as "Advertisements" or "Sponsored links." Also, avoid placing ads in locations where they might be confused with menu, navigation, or download links.
Less can be more
While our policies allow you to place 3 ad units, 3 link units, and 2 search boxes on each page of your site, placing the maximum number of ads on your page may make it look cluttered. If users can't find what they're looking for on your site, they may turn elsewhere for information.
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This is posted here for the viewers who can be benifited by theses policys and rules by Google.com for their adsense accounts.