In the recent post on reclaimed floorboards there seemed to be some confusion over when tongue & groove floorboards became availiable. My house which consists of an 18th century stone cottage modified in the 1850s (extended backwards and upwards) contains original wooden floors from both periods. The 18th century boards being 9inch wide T&G and the later being around 5inch T&G. I researched through some of my literature on period houses and apparently the first floorboards were hand sawn and as such had a "nearly" square edge. Mechanisation in the 18th century enabled production of "proper" square edge boards and T & G. Both were introduced at roughly the same time.
So there you have it - T & G has been around as long as the machine produced square edge boards - 18th Century.
Andy