Visa has made an undisclosed investment in payments startup Square in exchange for a seat on the firm’s board of, Reuters reports. Square is a portable credit card reader that was created by Twitter co-founder Jack Dorsey. It allows iPhone, iPod touch, or iPad owners to charge for goods and services on-the-go, but came under some scrutiny earlier this year when VeriFone’s CEO slammed the product in a blog post. On April 21st, Apple and Square announced that the Square credit card reader was available online and in Apple Stores nationwide for just $9.95.