The term “racist dog whistle” is not new. But President Donald Trump may represent the perfection of “dog-whistle politics,” using phrases, symbols and tweets to create plausible deniability whenever his policies seem too overtly racist.

The phrase has become so cliche that we may forget that a dog whistle is more than a style of speaking. It is a specific technology: a manufactured object constructed to make use of both physical laws (acoustics) and biological ones (the hearing abilities of animals). Dog whistles work by producing sounds at frequencies that canines can hear but humans cannot. Over time, the technology has become a stand-in for something uttered publicly but heard privately — a secret signal. And yet the fact that this phrase gets used so often is proof that the metaphor fails. Each time someone calls out acts of coded racism as a “dog whistle,” that’s proof that the “wrong” audience heard it.

There are several technologies that can transmit messages to exclusive audiences, from WWII-era Enigma machines to encrypted WhatsApp messages. So why has the dog whistle become the metaphor of choice? The racist roots of dog whistle technology itself point to some answers.

The device we now think of as a dog whistle was designed by Francis Galton, whose most famous work was inventing the term “eugenics” and creating a science of racial differences and race “improvement.” By the 1870s, Galton had developed a whistle whose tube length could be precisely adjusted by a plug that screwed into its base — changing the tone it produced. Galton tested his whistle at London’s Zoological Gardens, placing it near different animals and observing their reactions to blasts emitted at different frequencies. He concluded that cats were best at hearing extremely high-pitched sounds, ascribing the feline ability to his cousin Charles Darwin’s theory of natural selection, arguing that cats had evolved this shrill skill to detect squeaking mice.

But Galton’s primary interest was developing a scientific study of human differences. In 1869, he published “Hereditary Genius,” a book arguing that human intelligence and other mental attributes were biologically inherited. Galton disregarded “pretensions of natural equality” and concluded that high achievers tended to have children, brothers or cousins who were also well accomplished. Galton carried this “science” to an appallingly racist conclusion, asserting that high-achieving Black people could never rank higher than the average Anglo-Saxon, and that statistically the race was inferior.

Galton’s whistle was a scientific instrument designed to test his hypothesis that differences among human races were the result of inheritance rather than environment. If race differences (based on his flawed method of measuring intelligence and accomplishment) were the result of an evolutionary process, then they wouldn’t disappear by providing equal opportunities and resources. Galton made his trips to the zoo to reinforce his idea that biological differences were both inherited and attributable to evolution. Animal tests were key to his scientific racism.

In Galton’s day, there were already “dog whistles” in use throughout Britain and the United States, primarily for hunting. These produced tones at frequencies most humans could detect, but their sound could reach greater distances than human speech; they aided dogs at hunting foxes or birds. These audible whistles and Galton’s whistle coexisted side by side for decades, as distinct technologies.

The use of the original “dog whistles” was shaped by racial dynamics in the United States. Hunting dogs didn’t just pursue foxes or birds. Before the Civil War, they were frequently used to track enslaved Black people. Dogs trained to hunt humans were highly prized among enslavers and the slave catchers they employed.

Even after slavery’s abolition, dogs continued to reinforce the racial politics of the Jim Crow South. Trained dog packs, used in activities like fox hunting, became a manifestation of White Southern elites’ pretensions toward aristocracy and nostalgia for the era of slavery.

It was not until about 1940 that the dog whistle, a tool for signaling and training hunting dogs, and Galton’s ultrasonic whistle — mainstay of the psychology lab — were combined into a single technology. The first patent was filed for a dog whistle “of [sufficiently] high frequency so that it may be heard by a dog but not heard by the human ear.”

The “silent” dog whistle changed which kinds of dogs were being whistle-trained. No longer were whistles primarily a tool for hunting over long distances. They were for suburban dogs — an increasingly salient feature of the racially segregated and suburbanizing American landscape. As ads for Purina whistles promised, these couldn’t disturb your neighbors.

Silent dog whistles also became a tool for training police dogs. Though connections between dogs and policing in America go back to the days of slave catchers, the role of dogs in modern policing gained wider use in the 1950s. According to one 1961 study, “the public has fully accepted the canine corps. There were only three complaints reported, one not described and the other two stemming from the feeling among Negroes that dogs are used in their areas disproportionately to the need for them.” The overpolicing of Black neighborhoods — including with dogs — was rooted in ideas about criminality, race and intelligence and morals that go straight back to Galton and the psychologists he influenced.

The introduction of dogs to modern policing took hold just in time for the animals to feature in iconic and violent images of the struggle for civil rights, including police dogs attacking peaceful Black protesters in Birmingham, Ala., in 1963. President Trump evoked this scene earlier this year when he threatened Black Lives Matter protesters with “vicious dogs,” a comment that only avoided the “racist dog whistle” label because its motives seemed obvious.

It was in this context that the silent dog whistle, an invention that unified racist scientific equipment with racist cultures of dog hunting, became a technology that facilitated violent opposition to civil rights.

But even during the Civil Rights era, this fraught tool had yet to become a metaphor for a broader category of secret signals or exclusionary communication. In a 2008 book, William Safire claimed that the metaphorical use of dog whistle began 20 years prior in a column by then-Washington Post polling director Richard Morin, who asserted that “dog whistle effect” was a term of art among public opinion researchers. Morin’s column was written just weeks after the broadcast of the infamous Willie Horton campaign ad, perhaps the clearest example of a racist dog whistle in American politics before Trump.

Yet the metaphor really only became common parlance in President Barack Obama’s second term, and has skyrocketed since the 2016 election.

When a technology becomes a metaphor, we recycle knowledge about how it works and what effects it has on us to articulate another part of our lives. Our needs and desires, how something is sold to us and what it says about us when we use technology all help give the metaphor meaning.

If the dog whistle had no historical or cultural baggage, then it wouldn’t be useful as a metaphor; we have to define what makes a technology meaningful when discussing something beyond its literal usage. And others must share that definition for the metaphor to have any power of communication.

In fact, one of the ways that dog whistle communication functions is by invoking metaphors that seem literal-minded and straightforward to many, while conveying deeper implications to those few who are in the know.

People may not be explicitly aware of the racist history of dog whistles, but they’ve tacitly accepted a cultural landscape where this tool evolved a particular usage. The dog whistle became a useful metaphor for political speech acts, not simply because it’s a technology of selective communication, but because it reflects a history of racial power combined with scientific authority.

When people criticize racist dog whistles, they’re not just objecting to a specific coded speech act; they’re calling out a system that makes such acts of coded power possible. And when people knowingly use dog whistles to spread racist messages, they show contempt not just for the people they’re speaking past, but to the people they’re speaking to. Even if those who leap at the dog whistle revel in the insider knowledge that makes them the intended audience, this still places them not in the role of the master, but the dog.

Adam R. Shapiro is a historian of science and technology. He’s the author of “Trying Biology: The Scopes Trial Textbooks, and the Antievolution Movement in American Schools.”