Today various kinds of composite materials are used for subflooring because they are available and usually cheaper. However before these "improvements" in building technology, a T&G subflooring would have common brcause it was available and often cheaper than the newer materials like plywood.

The lower cost of the material was a result of knots and other defects not suitable for finish flooring. But the T&G-ness of the material you are seeing doesn't contraindicate its original use as sub-floor. Where you can see long runs of it, does it look particularly knotty? Are they just the common flat sawn boards? Those are the clues.

FWIW, 4 1/4" wide boards don't sound prettier than what you already described that you have. And they are not what I would describe as wide-board, which at least where I am in the Northeast is 8" to 16". These wider boards, at least in older houses (early/mid 19th c), were used because they were available and "cheaper" in terms of the amount of effort and time needed to work them up from raw timber. Making T&G edges was laborious before machine tools were common. The width you're describing is commonly used as subflooring here for those projects done in the late 19th-20th c. because by then the T&G edging was done by machines, so it was inexpensive and a way to use up lower grade materials.

When studying our old buildings, we always need to keep in mind that people have always chosen on the basis of availability and cost for the perceived value. Just the same factors that govern our own choices standing in the aisles of Home Depot.

Some of the things we old-house owners treasure now were often simply ordinary material choices of standard housing. It behooves us to be careful not to retrospectively gussy-up our buildings to a false degree of historical "accuracy". Certainly by the late-19th to 20th c, machine-made T&G subflooring would have been the norm because of its cost and functional stability as a substrate for a nicer looking finish floor. Builders and owners of that time would have scorned as cheap-looking and old-fashioned the idea that exposed wideboards were the ne plus ultra of a good-looking building.

L

Another clue would be if the planks are laid at an angle to the joists.

Laying planks at 45 degrees was a common sub-floor practice.

Ours (which, granted, is 30 years older than that) is T&G and indicated as such in our blueprints. It's Douglas fir instead of oak, though. A lot of people in our neighborhood pull up carpeting and tile and such and just use the subfloor as the main floor in parts of the house without a second layer of wood---it's actually quite pretty (though soft).

Thanks for all the great info! I am very intrigued by the idea of using a subfloor as the actual floor. We were going to install hardwood floors throughout the "new" (1940s) addition to the house, but maybe now we can avoid that expense and save some money by exposing and cleaning up the subfloor. The underside of the tongue and groove boards are fairly rough and the wood looks rather soft, but I don't see too many knots anywhere and aside from the wood's current texture, it looks like it would sand/finish up nicely.