Sherlock Holmes and Dr Watson. Gin and tonic. Stiffness and upper lips. Some things just go together in Britain: They always have and they always will.
Prince William and Prince Harry's relationship, for years, looked like it belonged on that list too. From the time Harry was born in 1984, they were joined at the junior culotte. (Diana's penchant for flouncy Victorian clothes for her kids made them look like they were perennially auditioning for an amateur production of Hello Dolly.) At the time, Prince Charles is reported to have said: "William has taken to the new baby like a duck to water. He's been climbing in and out of Harry's cot."
And if you change "cot" to "life", they've been doing more or less the same thing for the last three decades. (Thankfully they have ditched the culottes though.)
Their mother's devastating death in 1997 bonded them in a way that few siblings, thankfully, experience. Adulthood, military tours, the demands of royal duty and even the arrival of one Kate Middleton on the scene did nothing to dull their abiding attachment to one another.
But implicit in that relationship was an inherent inequity. One of the boys would be elevated to being a king and the titular ruler of 2.4 billion people. The other was going to spend decades opening municipal sports centres and putting up with jokes about being "the spare".
This was a fact Harry was keenly aware of, even when he was only five years old, according to the Princess of Wales' former protection officer Ken Wharfe. "It was a Friday night and we were driving to Highgrove. Diana was in the front and Olga (William and Harry's nanny) was in the back with the boys who were being raucous," Mr Wharfe told royal biographer Katie Nicholl.
"Olga told them to be quiet and stop it and William answered back. Olga told him: 'Don't be rude', and Harry piped up in the back, 'It doesn't matter anyway because William is going to be king'. It was extraordinary; even at that age he knew."
Like it or not, from such a young age, Harry knew he had been cast in the role of part jester, part sidekick, yet constant helpmate for William, as he in turn faced the slow but inexorable pull of destiny and duty.
And for years that was exactly what happened. Royal biographer Penny Junor has told the (London) Times: "Those boys were so close and their plans were that they would remain close, that Harry would be William's wingman."
Until, something changed for Harry, and he stepped out of the supporting actor role.
So, the rakish red-headed Prince started the truly inspiring Invictus Games. He daringly trekked to the South Pole. And through all of this, and more, he started to forge an identity and royal career that was totally independent of his brother.
That shift has only intensified in the past 18 months. Whether that is down to the arrival of Meghan Markle as a full-time presence in his life or Harry simply maturing as an adult we don't know. But what is unquestionable is Harry is no longer playing second banana.
In the past year, he and wife Meghan have ramped up their efforts to build a brand as passionate, modern activists who are using their platform for global good. Nothing about that says "wingman".
Whether this paradigm shift is to blame for the "Fab Four's" recent frostiness is probably only known to the Sussexes, the Cambridges and some footmen with excellent hearing.
It's hard to imagine this fraternal overhaul would be anything but a challenging and upsetting situation for Wills to find himself in. Especially given Brand Harry (and Meghan) is to some extent eclipsing the years of dedicated service he and wife Kate have put in.
However, what is clear is the Wills'n'Harry double act we have grown to love is well and truly dead.
The enduring affinity the Wales boys have shared since those cot-climbing days has clearly ebbed away. Quite what comes next, we don't know. And maybe that goes for Wills and Harry too.