Just as there's something special about a batsman completing a century - and with it the argument that it's not so different from, say, 99, but seems to matter far more - so too the triple decker.
After all, there's only been 26 batsman in 139 years of test cricket to have made it that far.
Only four have done it more than once - inevitably Don Bradman, cricket's batting legend, Sehwag and the lefthanded West Indians Chris Gayle and Brian Lara.
Lara's 400 against England in Antigua in 2004 is the highest individual score in tests, and a mark unlikely to be beaten but for one point: the speed at which batsmen gather their runs has quickened in the modern game.
Nair got his runs off 381 balls in 560 minutes, which equates to a day and a half's batting; Sehwag, by contrast, scored his 319 off 304 balls in 530 minutes on the same ground eight years ago.
No one comes close in the history of heavy scoring to Nair's needing just three test innings for his maiden triple. The closest was England's Len Hutton, who made his monumental 364 in 1938 at The Oval in his ninth innings.
Only two players have made a higher maiden test century than Nair -Garry Sobers' 365 not out against Pakistan at Kingston in 1957-58, and Australian Bob Simpson, who made 311 against England at Old Trafford in 1964.
Also remember the big scores generally belong to batsmen in the top four.
Nair, who batted at No 5, is an exception. Just two players have scored more runs at No 5 or lower in the order - former Australian captain Michael Clarke with 329 not out against India in Sydney in 2011-12; and Bradman with 304 against England at Leeds in 1934. McCullum's 302 slots in one run behind Nair.
The 26 triple bangers have been shared between six Australians, five Englishmen, four West Indians and Pakistanis, three Sri Lankans, two Indians, one South African and one New Zealander.
One claim Nair doesn't have is on age.
He was 25 years, 10 days yesterday. There are four men who got there at a younger age, a list headed by the dazzling Sobers, who was 21 years, 213 days when he had his marathon against Pakistan.
McCullum, incidentally, was 32 years, 140 days at the Basin in 2014.
The oldest to 300 was England's Andrew Sandham, who was a remarkable 39 years 271 days when he made 325 against the West Indies at Jamaica in 1930.
That was his final test; by contrast Nair is just starting out on his career as the latest in India's production line of remarkable test batting talents.
And consider he was only playing because two regular batting picks, Ajinkya Rahane and Rohit Sharma, were ruled out.
Careers, and lives, can change on little elements like that.
And who would be an Indian selector right now.
Cricket's youngest test triple centurymakers:
21 years 213 days: Garry Sobers (West Indies) 365 not out v Pakistan, Jamaica 1957-58
21 years 318 days: Don Bradman (Australia) 304 v England, Leeds, 1930
22 years 58 days: Len Hutton (England) 364 v Australia, The Oval, 1938
23 years 27 days: Hanif Mohammad (Pakistan) 337 v West Indies, Barbados 1957-58
24 years 349 days: Brian Lara (West Indies) 375 v England, Antigua, 1994
25 years 10 days: Karun Nair (India) 303 not out v England, Chennai, 2016
