After the Tokugawa made the final steps of unifying Japan, swordsmiths adopted more clear traditions of signing swords and dating them became much more common. The information they left behind and the fact that we’re dealing with “near history” makes it easier to understand swordsmith lineages.
When it gets into the Muromachi period and earlier, things get a bit more murky. Many signatures were lost, dates are few and far between, and period specific references can contradict each other.
In the modern period, with swords accessible to everyone and importantly with the work of the NBTHK passing Juyo blades and publishing them, the picture has become more clear. We owe a lot to Fujishiro Yoshio who’s work in the early 1900s on reference materials is more often right than wrong. So I’ll start this discussion with some of his general thoughts on this matter of one or two generations.