Screen shot by CNET

Twitter and American Express today launched a partnership that will allow Amex card holders to make certain purchases simply by tweeting.

According to an Amex Web page, the arrangement works like this. Amex cardholders first sync their card with Twitter. Then, when they come across products that are eligible to purchase under the plan -- products that American Express will promote through a Twitter feed -- they simply send out a tweet that includes a special hashtag. Amex will then send them an @-reply with a confirming hashtag. Finally, the buyer has to send out a second tweet with the special hashtag within 15 minutes.

This isn't the first time that Twitter and Amex have worked together. Last March, the two companies unveiled an initiative that would allow cardholders to use hashtags to save money on some purchases at Whole Foods.

Twitter and Amex also inked a deal a year ago under which the social network agreed to let cardholders and merchants who accept Amex purchase ads.