The second Amazon Go location opened in Chicago today, and a third is coming soon, showing that the tech giant has targeted the Windy City as the second city for further expansion of the cashier-less convenience store concept.

The location at 144 S. Clark St. is the fifth Amazon Go store overall. The 1,200-square-foot store is the smallest one yet.

Amazon didn’t give an opening date for the third store at 500 W. Madison St., saying only that it is “coming soon.” This location will be a little bigger at 1,700 square feet.

The first Amazon Go store in Chicago opened last month, following up on a plan disclosed earlier this year to expand the concept outside Seattle. Chicago and San Francisco were the first expansion cities named, and Amazon confirmed a future location in New York City last month.

After spending years developing the technology behind the store, and another year testing it with Amazon employees, the tech giant is speeding up the pace of Amazon Go expansion. In late September reports surfaced that Amazon is considering opening up to 3,000 Amazon Go locations by 2021.

The original store opened in January in the base of Amazon’s Day One officer tower. The second store at the Madison Centre office building in downtown Seattle opened in late August. Store number three opened a week later in one of Amazon’s office buildings at 300 Boren Ave. N. in the South Lake Union neighborhood.

The store relies heavily on the Amazon Go smartphone app. Customers scan a unique QR code within the app before passing through a set of glass doors, similar to the gates Amazon employees go through when entering their office buildings every morning. A vast array of overhead cameras and weight sensors in the shelves automatically track what people pick up and take from the store.

When customers leave, they just walk out. Amazon Go’s systems automatically debit their accounts for the items they take, sending the receipt to the app. By logging shoppers in at the entrance, then tracking their actions in the store, the system eliminates the need for traditional checkout registers and checkout workers along with them.

Each store features a mix of grab-and-go breakfast, lunch and dinner options such as salads and sandwiches. Also prominent in Amazon Go stores are Amazon Meal Kits, items from local producers and some grocery essentials.

In addition to the expansion of Amazon Go, the company recently debuted a new retail concept in New York City. Amazon 4-star features the highest rated items online, as well as new or trending items.

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