diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index c91a647a2f67b0d..350d70061d4624e 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -342,10 +342,12 @@ def tell(self):
         return self.seek(0, 1)
 
     def truncate(self, pos=None):
-        """Truncate file to size bytes.
+        """Resize stream to at most 'pos' bytes.
 
-        Size defaults to the current IO position as reported by tell().  Return
-        the new size.
+        The position in the stream is left unchanged.  The size
+        defaults to the current IO position as reported by tell().  If
+        the stream's size is increased, the contents of the new file
+        area are undetermined.  Returns the new size.
         """
         self._unsupported("truncate")
 
@@ -1670,10 +1672,12 @@ def tell(self):
         return os.lseek(self._fd, 0, SEEK_CUR)
 
     def truncate(self, size=None):
-        """Truncate the file to at most size bytes.
+        """Resize stream to at most 'size' bytes.
 
-        Size defaults to the current file position, as returned by tell().
-        The current file position is changed to the value of size.
+        The position in the stream is left unchanged.  The size
+        defaults to the current IO position as reported by tell().  If
+        the stream's size is increased, the contents of the new file
+        area are undetermined.  Returns the new size.
         """
         self._checkClosed()
         self._checkWritable()
@@ -1778,7 +1782,13 @@ def write(self, s):
         self._unsupported("write")
 
     def truncate(self, pos=None):
-        """Truncate size to pos, where pos is an int."""
+        """Resize stream to at most 'pos' characters.
+
+        The position in the stream is left unchanged.  The size
+        defaults to the current IO position as reported by tell().  If
+        the stream's size is increased, the contents of the new file
+        area are undetermined.  Returns the new size.
+        """
         self._unsupported("truncate")
 
     def readline(self):
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index ba33f8c50d6aade..a52979357d2530b 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -552,15 +552,17 @@ _io.BytesIO.truncate
     size: Py_ssize_t(accept={int, NoneType}, c_default="self->pos") = None
     /
 
-Truncate the file to at most size bytes.
+Resize stream to at most 'size' bytes.
 
-Size defaults to the current file position, as returned by tell().
-The current file position is unchanged.  Returns the new size.
+Position in the stream is left unchanged.  Size defaults to
+the current IO position as reported by tell().  If the stream's
+size is increased, the contents of the new file area are
+undetermined.  Returns the new size.
 [clinic start generated code]*/
 
 static PyObject *
 _io_BytesIO_truncate_impl(bytesio *self, Py_ssize_t size)
-/*[clinic end generated code: output=9ad17650c15fa09b input=423759dd42d2f7c1]*/
+/*[clinic end generated code: output=9ad17650c15fa09b input=0f69785dcc22737e]*/
 {
     CHECK_CLOSED(self);
     CHECK_EXPORTS(self);
diff --git a/Modules/_io/clinic/bytesio.c.h b/Modules/_io/clinic/bytesio.c.h
index 5c155cb3066e3b5..f31e34551b63e47 100644
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -314,10 +314,12 @@ PyDoc_STRVAR(_io_BytesIO_truncate__doc__,
 "truncate($self, size=None, /)\n"
 "--\n"
 "\n"
-"Truncate the file to at most size bytes.\n"
+"Resize stream to at most \'size\' bytes.\n"
 "\n"
-"Size defaults to the current file position, as returned by tell().\n"
-"The current file position is unchanged.  Returns the new size.");
+"Position in the stream is left unchanged.  Size defaults to\n"
+"the current IO position as reported by tell().  If the stream\'s\n"
+"size is increased, the contents of the new file area are\n"
+"undetermined.  Returns the new size.");
 
 #define _IO_BYTESIO_TRUNCATE_METHODDEF    \
     {"truncate", (PyCFunction)_io_BytesIO_truncate, METH_FASTCALL, _io_BytesIO_truncate__doc__},
@@ -444,4 +446,4 @@ _io_BytesIO___init__(PyObject *self, PyObject *args, PyObject *kwargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=9ba9a68c8c5669e7 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=934f685bde756d6d input=a9049054013a1b77]*/
diff --git a/Modules/_io/clinic/fileio.c.h b/Modules/_io/clinic/fileio.c.h
index 8a9958aaecf0de9..048fa8716d5c198 100644
--- a/Modules/_io/clinic/fileio.c.h
+++ b/Modules/_io/clinic/fileio.c.h
@@ -322,10 +322,12 @@ PyDoc_STRVAR(_io_FileIO_truncate__doc__,
 "truncate($self, size=None, /)\n"
 "--\n"
 "\n"
-"Truncate the file to at most size bytes and return the truncated size.\n"
+"Resize stream to at most \'size\' bytes.\n"
 "\n"
-"Size defaults to the current file position, as returned by tell().\n"
-"The current file position is changed to the value of size.");
+"Position in the stream is left unchanged.  Size defaults to\n"
+"the current IO position as reported by tell().  If the stream\'s\n"
+"size is increased, the contents of the new file area are\n"
+"undetermined.  Returns the new size.");
 
 #define _IO_FILEIO_TRUNCATE_METHODDEF    \
     {"truncate", (PyCFunction)_io_FileIO_truncate, METH_FASTCALL, _io_FileIO_truncate__doc__},
@@ -373,4 +375,4 @@ _io_FileIO_isatty(fileio *self, PyObject *Py_UNUSED(ignored))
 #ifndef _IO_FILEIO_TRUNCATE_METHODDEF
     #define _IO_FILEIO_TRUNCATE_METHODDEF
 #endif /* !defined(_IO_FILEIO_TRUNCATE_METHODDEF) */
-/*[clinic end generated code: output=a8796438c8b7c49a input=a9049054013a1b77]*/
+/*[clinic end generated code: output=b0ef426afce9a6c3 input=a9049054013a1b77]*/
diff --git a/Modules/_io/clinic/stringio.c.h b/Modules/_io/clinic/stringio.c.h
index 72b2540d47aab98..1bbf30ce7e0b55d 100644
--- a/Modules/_io/clinic/stringio.c.h
+++ b/Modules/_io/clinic/stringio.c.h
@@ -103,11 +103,12 @@ PyDoc_STRVAR(_io_StringIO_truncate__doc__,
 "truncate($self, pos=None, /)\n"
 "--\n"
 "\n"
-"Truncate size to pos.\n"
+"Resize stream to at most \'pos\' characters.\n"
 "\n"
-"The pos argument defaults to the current file position, as\n"
-"returned by tell().  The current file position is unchanged.\n"
-"Returns the new absolute position.");
+"The position in the stream is left unchanged.  The size defaults to\n"
+"the current IO position as reported by tell().  If the stream\'s size\n"
+"is increased, the contents of the new file area are undetermined.\n"
+"Returns the new size.");
 
 #define _IO_STRINGIO_TRUNCATE_METHODDEF    \
     {"truncate", (PyCFunction)_io_StringIO_truncate, METH_FASTCALL, _io_StringIO_truncate__doc__},
@@ -286,4 +287,4 @@ _io_StringIO_seekable(stringio *self, PyObject *Py_UNUSED(ignored))
 {
     return _io_StringIO_seekable_impl(self);
 }
-/*[clinic end generated code: output=73c4d6e5cc3b1a58 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=64a903b7c3c45564 input=a9049054013a1b77]*/
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index 0d5bf3b22114e5e..427c1b59bb64385 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -979,15 +979,17 @@ _io.FileIO.truncate
     size as posobj: object = NULL
     /
 
-Truncate the file to at most size bytes and return the truncated size.
+Resize stream to at most 'size' bytes.
 
-Size defaults to the current file position, as returned by tell().
-The current file position is changed to the value of size.
+Position in the stream is left unchanged.  Size defaults to
+the current IO position as reported by tell().  If the stream's
+size is increased, the contents of the new file area are
+undetermined.  Returns the new size.
 [clinic start generated code]*/
 
 static PyObject *
 _io_FileIO_truncate_impl(fileio *self, PyObject *posobj)
-/*[clinic end generated code: output=e49ca7a916c176fa input=9026af44686b7318]*/
+/*[clinic end generated code: output=e49ca7a916c176fa input=b60ed73a8ce11d96]*/
 {
     Py_off_t pos;
     int ret;
diff --git a/Modules/_io/iobase.c b/Modules/_io/iobase.c
index 0c329bff840bd3c..b696290444ca05d 100644
--- a/Modules/_io/iobase.c
+++ b/Modules/_io/iobase.c
@@ -118,10 +118,12 @@ _io__IOBase_tell_impl(PyObject *self)
 }
 
 PyDoc_STRVAR(iobase_truncate_doc,
-    "Truncate file to size bytes.\n"
+    "Resize stream to at most 'size' bytes.\n"
     "\n"
-    "File pointer is left unchanged.  Size defaults to the current IO\n"
-    "position as reported by tell().  Returns the new size.");
+    "Position in the stream is left unchanged.  Size defaults to\n"
+    "the current IO position as reported by tell().  If the stream's\n"
+    "size is increased, the contents of the new file area are undetermined.\n"
+    "Returns the new size.");
 
 static PyObject *
 iobase_truncate(PyObject *self, PyObject *args)
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 718b1ac1d82db2b..9a7483e744a8d0c 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -435,16 +435,17 @@ _io.StringIO.truncate
     pos as size: Py_ssize_t(accept={int, NoneType}, c_default="self->pos") = None
     /
 
-Truncate size to pos.
+Resize stream to at most 'pos' characters.
 
-The pos argument defaults to the current file position, as
-returned by tell().  The current file position is unchanged.
-Returns the new absolute position.
+The position in the stream is left unchanged.  The size defaults to
+the current IO position as reported by tell().  If the stream's size
+is increased, the contents of the new file area are undetermined.
+Returns the new size.
 [clinic start generated code]*/
 
 static PyObject *
 _io_StringIO_truncate_impl(stringio *self, Py_ssize_t size)
-/*[clinic end generated code: output=eb3aef8e06701365 input=5505cff90ca48b96]*/
+/*[clinic end generated code: output=eb3aef8e06701365 input=f2b6a674a34b14a5]*/
 {
     CHECK_INITIALIZED(self);
     CHECK_CLOSED(self);