At energy levels where predictions from string theory and general relativity match up, D-branes and p-branes are equivalent. Though Polchinski was aware of Strominger’s work on p-branes — they discussed their projects over lunch regularly — both scientists thought that the two types of branes were distinct. Part of Polchinski’s 1995 work on branes included the realization that they were actually one and the same object.
It might seem odd that this hadn’t occurred to either of the men before 1995, but there was no reason to expect that the two types of branes would be related to each other. To a layman, they sound basically the same — multidimensional surfaces existing in a 10-dimensional space-time. Why wouldn’t you at least consider that they’re the same things?
Well, part of the reason may be based on the specific nature of scientific research. When you’re working in a scientific field, you are quite specific about the questions you’re asking and the ways in which you’re asking them.
Polchinski and Strominger were asking different questions in different ways, so it never occurred to either of them that the answers to their questions might be the same. Their knowledge blinded them from seeing the commonalities. This sort of tunnel vision is fairly common and part of the reason why sharing research is so encouraged within the scientific community.
Similarly, for a laymen, the dramatic differences between these two types of branes are less clear. Just as someone who doesn’t study much religion may be confused by the difference between Episcopalian and Catholic theological doctrines, to a priest of either religion the differences are well-known, and the two are seen as extremely distinct.
In the case of branes, though, the laymen would have had clearer insight on the issue than either of the experts. The very details that made D-branes and p-branes so intriguing to Polchinski and Strominger hindered their ability to see past the details to the commonalities — at least until 1995, when Polchinski finally saw the connection.
Because of equivalence, both D-branes and p-branes are typically just referred to as branes. When referencing their dimensionality, the p-brane notation is usually the one used. Some physicists still use the D-brane notation because there are other types of branes that physicists talk about.