Every floor on the first floor of my house squeaks no matter where one steps on the floor. Looking from the basement it looks like there are thick wood slats nailed to the beams and a sub floor above the slats and then the wood floor.
JFL ... perhaps you could provide some pictures as to what you are trying to describe.
Squeaky floors can be caused by a few things and be common with hardwood flooring but also be present with carpeting and other coverings as well.
Depending on the age of the home the fasteners for the sub floor may have have worked loose over the years and in cases may be improperly installed. In other words if the fasteners are nails they have worked loose from the wood used for the sub floor drying over time and the wood is rubbing along the nail causing the squeak noise you hear.
Other cases the nails may have missed being driven into the joists below so the wood sub floor is rubbing on the joist because it's loose in those spots.
Another cause of fasteners working loose is the floor joists have a bounce to them. This is an up and down flex that occurs and over time this will contribute to the fasteners working loose.
If the floors are hardwood then they will loosen over the years with the natural expanding an contracting that occurs with the wood.
Canuk pretty well covered it. In very early homes the joists were undersized especially for the loads of modern living.
If the problem is flex in your floor I am in the process of marketing a bracket and threaded rod system to take the flex out of a floor and still not interfere with the space below. It really worked on my house which used 2X8 floor joists and to long of a span. Now you can walk across the floor and nothing moves.