Through U.S. history, both technology and the law have evolved to allow the President of the United States to speak directly to the American people in times of emergency through their phones.
Given Donald Trump's prolific use of social media platforms, like Twitter, it's worth examining how the president-elect may use this technology to communicate with citizens.
Let's start with some background.
In 1951, President Harry Truman signed legislation establishing the first national warning system, called CONELRAD (Control of Electromagnetic Radiation.) Upon atomic attack by the Soviets, it would let the president broadcast warnings and civil defense instructions on two alternating frequencies, which were changed at random to prevent Russian bombers from using them as homing beacons.
That system was replaced in 1963 by the Emergency Broadcast System, which was in turn replaced in 1994 by the Emergency Alert System. Both allowed the president to address the nation within minutes of a nuclear attack or national catastrophe. Neither were ever used, even during the terrorist attacks of September 11, 2001, but were tested on a weekly basis, using a code of test patterns and screeches that routinely interrupted TV shows.
Criticism of that system's outdated technology led to its replacement in 2006 by Integrated Public Alert and Warning System (IPAWS), which allowed for broadcasting messages through the internet and on cell phone networks. One element of that was the Wireless Emergency Alert. WEAs are targeted messages that can be sent to every cell phone on a certain network, or throughout the country.
The 90-character messages come in three forms; alerts directly from the president, alerts about local threats to life and safety, and Amber Alerts.
Federal law allows the latter two to be blocked by individual users, but messages from the president legally can't be. This means that on January 20, President Trump will have the ability to send short messages to every phone in the country - with no ability to opt out.
How could a President Trump use this technology?
While it's easy to envision the Twitter-loving president bombarding a captive audience with insults directed toward whoever's out of favor with the administration, sending a WEA isn't quite that simple.
As part of the country's emergency alert network, IPAWS usage is administered by FEMA, who presumably would have to look at any message sent over its system. And since FEMA's head reports to the head of Homeland Security, it's likely DHS would have approval over a potential message as well.
Beyond that, Trump would have to be trained in how to use IPAWS. Trump could theoretically overrule both FEMA and DHS if sending the message was deemed important enough, but as long as he's got Twitter, it's likely not worth the effort.
While countless WEA's have been sent due to disasters or Amber Alerts, no president has issued one. The closest the system has come to its intended usage was New York City's use of a WEA message sent to millions of people in the hunt for the September Chelsea bomber.