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Google is now in the same-day delivery business. In San Francisco, some people affiliated with Google can buy a product, using their phones or computers, and have it delivered to their homes in a matter of hours.

Plans for the new service have been under way for more than a year. But it recently went live for some Google employees and their friends, according to two people briefed on the service who were not authorized to discuss it because Google has not yet publicly introduced it. At least one national apparel chain is involved, one of these people said. A Google spokesman, Nate Tyler, declined to comment.

Google is just one company tackling same-day delivery. So are Wal-Mart Stores, Amazon.com, eBay and the United States Postal Service.

Though the service propels Google into commerce, the company does not intend to operate warehouses or a shipping service but to team up with retailers and delivery companies. Several San Francisco retailers, including national chains, are participating in the program already.

For shoppers, the service means they can avoid the trouble of driving to the store and some of the wait for items ordered online.

Same-day delivery could help physical retailers, which have been under siege from e-commerce companies that offer the convenience of shopping without leaving home. But online retailers offering same-day delivery could make life even harder for physical retailers, because letting people own something the same day has become physical retailers’ biggest remaining advantage.

The reasons that Google is interested in same-day delivery are less obvious.

Retail ads are a huge portion of Google’s business, but they are under threat from companies like Amazon, where shoppers increasingly go to search for products, bypassing Google. Also responding to the threat from Amazon, Google recently tried to improve its comparison shopping service by charging retailers to list their products there. It says retailers are more likely to list accurate and up-to-date items if they are paying.

Additionally, Google has been trying to bridge the gap between the digital and physical worlds to better understand and profit from mobile ads. On computers, Google and advertisers know if a user clicks on an ad and visits or buys on another Web site. But they lose track of customers who look up a business or product on their phone and then put their phone away, walk into the store and buy something. Online ordering and delivery could help solve that problem.