In the early 1970s computer engineer Ray Tomlinson (1941–) noticed that people working at the same mainframe computer could leave one another messages. He imagined great utility of this communication system if messages could be sent to different mainframes. So he wrote a software program over the period of about a week that used file-transfer protocols and send-and-receive features. It enabled people to send messages from one mainframe to another over the Arpanet, the network that became the Internet. To make sure the messages went to the right system he adopted the @ symbol because it was the least ambiguous keyboard symbol and because it was brief.