Yet his vision for the US was most memorably expressed five years earlier, in a half-improvised speech to 250,000 people.

On a sweltering August afternoon in 1963, King stood before the Lincoln Memorial and addressed the nation. Referencing the Bible, the US Constitution and Shakespeare, he decried the "lonely island of poverty" experienced by the black population. "We cannot be satisfied," he said, as long as African-Americans face segregation, ghettos, police brutality and disenfranchisementDeprivation of the right to vote. It derives from a French word for free..

Dissatisfied with how the speech was going, King ditched his notes and began to speak personally. "I have a dream," he announced, describing his hope that races would join hands in a fair, tolerant society. Only thus, he concluded, would Americans become "free at last".

Half a century on from his death, King's legacy is secure - there is even a national holiday in his honour. But whether his dream has come true is still up for debate.

In many ways, it has. The law no longer allows segregation and protects everyone's right to vote. The US is home to Black billionaires and CEOs. Clarence Jones, one of King's closest friends, confesses that he did not think he would live to see a Black president.

When Barack Obama was elected, some declared that a "post-racial", prejudice-free era had dawned. Yet statistics suggest otherwise. Black citizens earn less, own less and die younger than the average American. They are more likely to be homeless or behind bars. By many measures, racial segregation in schools and neighbourhoods is growing.

Racial discontent is expressed in movements such as Black Lives Matter and phrases like "driving while Black", both inspired by police misconduct. Meanwhile, the white nationalism surrounding Donald Trump has many questioning whether the Obama presidency did much for race relations.

Racism is still rampant, say some. In many ways, it is getting worse; the wealth gap between White people and minorities is growing. The replacement of a Black president with a man endorsed by the Ku Klux KlanAn extremist white supremacy organisation which has existed since the 1860s. in 2016 says it all: as a nation, the US does not care enough about racial equality.

Let's not forget what has changed, say others. The events that led to King's speech, in which police dogs attacked civil rights protesters, would shock us now. Legal obstacles to equality are gone in many places. Society still has a way to go. Luckily, people are discussing racism more openly than ever.

Disenfranchisement: Being denied a right or privilege, especially the right to vote. Although African-American men were allowed to vote as of 1870, various methods - from literacy tests to pure intimidation - were used to prevent them from doing so. The Voting Rights Act of 1965 aimed to end these practices.

Billionaires: Of the 2,043 people on Forbes's list of global billionaires in 2017, only three are Black Americans: Oprah Winfrey, Michael Jordan and Robert Smith.

Driving while Black: This phrase is used to describe the perception that Black motorists are unfairly targeted by police officers.

Minorities: Between 1983 and 2013, the average Black household gained $18,000 in wealth; the average white family gained $301,000 (according to the report "The Ever-Growing Gap"). For various reasons, Black citizens were hit disproportionately hard by the financial crisis.

Ku Klux Klan: David Duke, the Klan's former leader, said that Trump was "the best of the lot" among the presidential candidates. Trump rejected the endorsement, but reluctantly.

Events: The riot in Birmingham, Alabama in May 1963.

What do we know?: In early April 1968, King was in Memphis, Tennessee, to support a sanitation workers' strike. On the 3rd, he gave a speech in a church, ending with words that seemed to anticipate his death: "I've seen the promised land. I may not get there with you." The following evening, a sniper's bullet struck and killed him as he stood on the balcony of his motel.

What do we not know?: Controversy still surrounds the assassination. James Earl Ray, a small-time criminal, confessed to it; his fingerprints were found on the rifle. He later retracted his confession, saying he was only part of a conspiracy behind the killing. King's own family believed that he was innocent, but he is widely considered to be the murderer. Although Ray was a known racist, his exact motivations remain unclear.