“The biggest friction has been places to pay,” said Jack Dorsey, the founder of Square, in an interview. He said that with so many different companies trying to get a piece of the market, paying with a phone has been a “fragmented” experience. But the Starbucks partnership should widen the use of Square specifically, he said.

Indeed, businesses of all kinds, including big companies like Google, Microsoft and Sprint and small start-ups like GoPago and Scvngr, are hoping to profit from mobile payments — if only they can figure out what kind of system appeals to consumers and merchants.

Google has developed a mobile wallet app that uses a technology called near-field communication, which allows a phone to communicate wirelessly with a nearby cash register. GoPago has an app that lets customers place an order before arriving in a store; it shows up on a tablet on the merchant’s counter. Square offers businesses software for the iPad that shows pictures of nearby customers who are using the Pay With Square app on their smartphone, so all they have to do is state their name to pay for an item.

Starbucks stores will begin accepting a less ambitious form of Square payments this fall, when customers will be able to show a Square bar code on their smartphone at the register. The companies expect 7,000 Starbucks stores in the United States to be hooked up to the new system before the holiday season.