Visa has purchased 10 percent of Square's trading shares according to documents seen by the WSJ. Square is best known for its smartphone-attached readers that make it easy for merchants (and even panhandlers) to accept credit cards. The company was started and is still headed by Twitter CEO Jack Dorsey. Though Square's Jack Dorsey revealed that Visa had taken a piece of it several years back, the credit card giant only recently revealed the amount of the stake. Visa recently launched Visa Developer, software that will help merchants accept Visa payments more easily.