A pager is a small electronic device that receives short messages, usually just a phone number or a brief text. Before cell phones became common in the late 1990s, doctors, emergency workers, and business people carried pagers clipped to their belts so they could be reached quickly when they weren't near a telephone.

Here's how it worked: someone would call a paging service and leave a number. The pager would beep or vibrate, and a phone number would appear on its tiny screen. The person with the pager would then find the nearest phone and call that number back. Many pagers could only receive messages, not send them, which seems incredibly limited now but was revolutionary technology at the time.

Pagers were most popular from the 1980s through the mid-1990s. They were essential for doctors who needed to know immediately when a patient had an emergency, or for parents who wanted to reach their teenagers. Some pagers could display short text messages using numbers as a code: for example, “143” meant “I love you” (one letter, four letters, three letters).

Today, pagers have been almost entirely replaced by smartphones, though some hospitals still use them because they're more reliable than cell phones in buildings with thick walls.