diff --git a/src/_pytest/capture.py b/src/_pytest/capture.py
index cb5b966c9..0ff583e04 100644
--- a/src/_pytest/capture.py
+++ b/src/_pytest/capture.py
@@ -1,4 +1,5 @@
 """Per-test stdout/stderr capturing mechanism."""
+import abc
 import collections
 import contextlib
 import io
@@ -270,6 +271,38 @@ class DontReadFromInput(TextIO):
 # Capture classes.
 
 
+class CaptureBase(abc.ABC, Generic[AnyStr]):
+    EMPTY_BUFFER: AnyStr
+
+    @abc.abstractmethod
+    def __init__(self, fd: int) -> None:
+        raise NotImplementedError()
+
+    @abc.abstractmethod
+    def start(self) -> None:
+        raise NotImplementedError()
+
+    @abc.abstractmethod
+    def done(self) -> None:
+        raise NotImplementedError()
+
+    @abc.abstractmethod
+    def suspend(self) -> None:
+        raise NotImplementedError()
+
+    @abc.abstractmethod
+    def resume(self) -> None:
+        raise NotImplementedError()
+
+    @abc.abstractmethod
+    def writeorg(self, data: AnyStr) -> None:
+        raise NotImplementedError()
+
+    @abc.abstractmethod
+    def snap(self) -> AnyStr:
+        raise NotImplementedError()
+
+
 patchsysdict = {0: "stdin", 1: "stdout", 2: "stderr"}
 
 
@@ -278,7 +311,7 @@ class NoCapture:
     __init__ = start = done = suspend = resume = lambda *args: None
 
 
-class SysCaptureBase:
+class SysCaptureBase(CaptureBase[AnyStr]):
     def __init__(
         self, fd: int, tmpfile: Optional[TextIO] = None, *, tee: bool = False
     ) -> None:
@@ -345,7 +378,7 @@ class SysCaptureBase:
         self._state = "started"
 
 
-class SysCaptureBinary(SysCaptureBase):
+class SysCaptureBinary(SysCaptureBase[bytes]):
     EMPTY_BUFFER = b""
 
     def snap(self) -> bytes:
@@ -363,7 +396,7 @@ class SysCaptureBinary(SysCaptureBase):
         self._old.buffer.flush()
 
 
-class SysCapture(SysCaptureBase):
+class SysCapture(SysCaptureBase[str]):
     EMPTY_BUFFER = ""
 
     def snap(self) -> str:
@@ -380,7 +413,7 @@ class SysCapture(SysCaptureBase):
         self._old.flush()
 
 
-class FDCaptureBase:
+class FDCaptureBase(CaptureBase[AnyStr]):
     def __init__(self, targetfd: int) -> None:
         self.targetfd = targetfd
 
@@ -478,7 +511,7 @@ class FDCaptureBase:
         self._state = "started"
 
 
-class FDCaptureBinary(FDCaptureBase):
+class FDCaptureBinary(FDCaptureBase[bytes]):
     """Capture IO to/from a given OS-level file descriptor.
 
     snap() produces `bytes`.
@@ -500,7 +533,7 @@ class FDCaptureBinary(FDCaptureBase):
         os.write(self.targetfd_save, data)
 
 
-class FDCapture(FDCaptureBase):
+class FDCapture(FDCaptureBase[str]):
     """Capture IO to/from a given OS-level file descriptor.
 
     snap() produces text.
@@ -550,10 +583,15 @@ class MultiCapture(Generic[AnyStr]):
     _state = None
     _in_suspended = False
 
-    def __init__(self, in_, out, err) -> None:
-        self.in_ = in_
-        self.out = out
-        self.err = err
+    def __init__(
+        self,
+        in_: Optional[CaptureBase[AnyStr]],
+        out: Optional[CaptureBase[AnyStr]],
+        err: Optional[CaptureBase[AnyStr]],
+    ) -> None:
+        self.in_: Optional[CaptureBase[AnyStr]] = in_
+        self.out: Optional[CaptureBase[AnyStr]] = out
+        self.err: Optional[CaptureBase[AnyStr]] = err
 
     def __repr__(self) -> str:
         return "<MultiCapture out={!r} err={!r} in_={!r} _state={!r} _in_suspended={!r}>".format(
@@ -577,8 +615,10 @@ class MultiCapture(Generic[AnyStr]):
         """Pop current snapshot out/err capture and flush to orig streams."""
         out, err = self.readouterr()
         if out:
+            assert self.out is not None
             self.out.writeorg(out)
         if err:
+            assert self.err is not None
             self.err.writeorg(err)
         return out, err
 
@@ -599,6 +639,7 @@ class MultiCapture(Generic[AnyStr]):
         if self.err:
             self.err.resume()
         if self._in_suspended:
+            assert self.in_ is not None
             self.in_.resume()
             self._in_suspended = False
 
@@ -621,7 +662,8 @@ class MultiCapture(Generic[AnyStr]):
     def readouterr(self) -> CaptureResult[AnyStr]:
         out = self.out.snap() if self.out else ""
         err = self.err.snap() if self.err else ""
-        return CaptureResult(out, err)
+        # TODO: This type error is real, need to fix.
+        return CaptureResult(out, err)  # type: ignore[arg-type]
 
 
 def _get_multicapture(method: "_CaptureMethod") -> MultiCapture[str]:
@@ -830,14 +872,18 @@ class CaptureFixture(Generic[AnyStr]):
     :fixture:`capfd` and :fixture:`capfdbinary` fixtures."""
 
     def __init__(
-        self, captureclass, request: SubRequest, *, _ispytest: bool = False
+        self,
+        captureclass: Type[CaptureBase[AnyStr]],
+        request: SubRequest,
+        *,
+        _ispytest: bool = False,
     ) -> None:
         check_ispytest(_ispytest)
-        self.captureclass = captureclass
+        self.captureclass: Type[CaptureBase[AnyStr]] = captureclass
         self.request = request
         self._capture: Optional[MultiCapture[AnyStr]] = None
-        self._captured_out = self.captureclass.EMPTY_BUFFER
-        self._captured_err = self.captureclass.EMPTY_BUFFER
+        self._captured_out: AnyStr = self.captureclass.EMPTY_BUFFER
+        self._captured_err: AnyStr = self.captureclass.EMPTY_BUFFER
 
     def _start(self) -> None:
         if self._capture is None:
@@ -922,7 +968,7 @@ def capsys(request: SubRequest) -> Generator[CaptureFixture[str], None, None]:
             assert captured.out == "hello\n"
     """
     capman: CaptureManager = request.config.pluginmanager.getplugin("capturemanager")
-    capture_fixture = CaptureFixture[str](SysCapture, request, _ispytest=True)
+    capture_fixture = CaptureFixture(SysCapture, request, _ispytest=True)
     capman.set_fixture(capture_fixture)
     capture_fixture._start()
     yield capture_fixture
@@ -950,7 +996,7 @@ def capsysbinary(request: SubRequest) -> Generator[CaptureFixture[bytes], None,
             assert captured.out == b"hello\n"
     """
     capman: CaptureManager = request.config.pluginmanager.getplugin("capturemanager")
-    capture_fixture = CaptureFixture[bytes](SysCaptureBinary, request, _ispytest=True)
+    capture_fixture = CaptureFixture(SysCaptureBinary, request, _ispytest=True)
     capman.set_fixture(capture_fixture)
     capture_fixture._start()
     yield capture_fixture
@@ -978,7 +1024,7 @@ def capfd(request: SubRequest) -> Generator[CaptureFixture[str], None, None]:
             assert captured.out == "hello\n"
     """
     capman: CaptureManager = request.config.pluginmanager.getplugin("capturemanager")
-    capture_fixture = CaptureFixture[str](FDCapture, request, _ispytest=True)
+    capture_fixture = CaptureFixture(FDCapture, request, _ispytest=True)
     capman.set_fixture(capture_fixture)
     capture_fixture._start()
     yield capture_fixture
@@ -1007,7 +1053,7 @@ def capfdbinary(request: SubRequest) -> Generator[CaptureFixture[bytes], None, N
 
     """
     capman: CaptureManager = request.config.pluginmanager.getplugin("capturemanager")
-    capture_fixture = CaptureFixture[bytes](FDCaptureBinary, request, _ispytest=True)
+    capture_fixture = CaptureFixture(FDCaptureBinary, request, _ispytest=True)
     capman.set_fixture(capture_fixture)
     capture_fixture._start()
     yield capture_fixture
diff --git a/testing/test_capture.py b/testing/test_capture.py
index fcd318347..26c1a5f74 100644
--- a/testing/test_capture.py
+++ b/testing/test_capture.py
@@ -1352,6 +1352,7 @@ def test_capsys_results_accessible_by_attribute(capsys: CaptureFixture[str]) ->
 
 def test_fdcapture_tmpfile_remains_the_same() -> None:
     cap = StdCaptureFD(out=False, err=True)
+    assert isinstance(cap.err, capture.FDCapture)
     try:
         cap.start_capturing()
         capfile = cap.err.tmpfile