It's customary to refer to Revelation chapters 2 and 3 as the "Letters to the Seven Churches".
As I've studied, and we've preached, our way through these chapters, I'm not convinced that's the best heading to give them.
In fact, the whole of Revelation is a single letter. It's an epistle, like Philippians is. We get that from verse 4 that begins:
“John, to the seven churches in the province of Asia: Grace and peace …”
The whole of Revelation is a letter. It was written to seven churches in Asia Minor.