Twitter posts already answer who, what and when. Now they will also tell people exactly where a post was written. On Monday, Twitter announced on its blog that Twitter Places is going live. Twitter users can tag their posts with their precise location and people can search for all the posts written from a specific location. Twitter’s founders have talked about the need to develop ways in which people can find the messages that are most relevant to them. This is a big step. If someone is walking into a concert and wants to know why the line is so long, for instance, they could search for the posts written from Madison Square Garden. If there are emergency vehicles outside a hotel, they could search posts written from the hotel. Location information could potentially be used by marketers who want to reach shoppers in a competitor’s store, for example, or people near a certain restaurant. Twitter does not intend to duplicate the services that location-based social networks, like Foursquare and Gowalla, were already providing. Indeed, Twitter integrated those two services into Twitter Places, so that if users click on a particular restaurant, they will see both Twitter posts and Foursquare and Gowalla check-ins from that restaurant. Twitter Places will be available on Twitter.com and its mobile site at first. Over the next week, Twitter will introduce it in 65 countries, through partnerships with TomTom, which makes navigation systems, and Localeze, the local search site.