diff --git a/Lib/_pyio.py b/Lib/_pyio.py
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -344,10 +344,12 @@
         return self.seek(0, 1)
 
     def truncate(self, pos=None):
-        """Truncate file to size bytes.
-
-        Size defaults to the current IO position as reported by tell().  Return
-        the new size.
+        """Resize stream to at most size bytes.
+
+        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.
         """
         self._unsupported("truncate")
 
@@ -1641,10 +1643,12 @@
         return os.lseek(self._fd, 0, SEEK_CUR)
 
     def truncate(self, size=None):
-        """Truncate the file 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.
+        """Resize stream to at most size bytes.
+
+        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.
         """
         self._checkClosed()
         self._checkWritable()
@@ -1749,7 +1753,13 @@
         self._unsupported("write")
 
     def truncate(self, pos=None):
-        """Truncate size to pos, where pos is an int."""
+        """Resize stream to at most size bytes.
+
+        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.
+        """
         self._unsupported("truncate")
 
     def readline(self):
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -584,15 +584,17 @@
     size as arg: object = 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, PyObject *arg)
-/*[clinic end generated code: output=81e6be60e67ddd66 input=11ed1966835462ba]*/
+/*[clinic end generated code: output=81e6be60e67ddd66 input=473d0e02cc30738b]*/
 {
     Py_ssize_t size;
 
diff --git a/Modules/_io/clinic/bytesio.c.h b/Modules/_io/clinic/bytesio.c.h
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -292,10 +292,12 @@
 "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_VARARGS, _io_BytesIO_truncate__doc__},
@@ -419,4 +421,4 @@
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=60ce2c6272718431 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=7276c906a3089120 input=a9049054013a1b77]*/
diff --git a/Modules/_io/clinic/fileio.c.h b/Modules/_io/clinic/fileio.c.h
--- a/Modules/_io/clinic/fileio.c.h
+++ b/Modules/_io/clinic/fileio.c.h
@@ -314,10 +314,12 @@
 "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_VARARGS, _io_FileIO_truncate__doc__},
@@ -364,4 +366,4 @@
 #ifndef _IO_FILEIO_TRUNCATE_METHODDEF
     #define _IO_FILEIO_TRUNCATE_METHODDEF
 #endif /* !defined(_IO_FILEIO_TRUNCATE_METHODDEF) */
-/*[clinic end generated code: output=dcbc39b466598492 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=ff25cfd6fc8e3d0f input=a9049054013a1b77]*/
diff --git a/Modules/_io/clinic/stringio.c.h b/Modules/_io/clinic/stringio.c.h
--- a/Modules/_io/clinic/stringio.c.h
+++ b/Modules/_io/clinic/stringio.c.h
@@ -103,11 +103,12 @@
 "truncate($self, pos=None, /)\n"
 "--\n"
 "\n"
-"Truncate size to pos.\n"
+"Resize stream to at most size bytes.\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.");
+"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_STRINGIO_TRUNCATE_METHODDEF    \
     {"truncate", (PyCFunction)_io_StringIO_truncate, METH_VARARGS, _io_StringIO_truncate__doc__},
@@ -283,4 +284,4 @@
 {
     return _io_StringIO_seekable_impl(self);
 }
-/*[clinic end generated code: output=f061cf3a20cd14ed input=a9049054013a1b77]*/
+/*[clinic end generated code: output=885c11830d8f3411 input=a9049054013a1b77]*/
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -988,15 +988,17 @@
     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=1957520f803e9c07]*/
 {
     Py_off_t pos;
     int ret;
diff --git a/Modules/_io/iobase.c b/Modules/_io/iobase.c
--- a/Modules/_io/iobase.c
+++ b/Modules/_io/iobase.c
@@ -127,10 +127,12 @@
 }
 
 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
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -462,16 +462,17 @@
     pos as arg: object = None
     /
 
-Truncate size to pos.
+Resize stream to at most size bytes.
 
-The pos argument defaults to the current file position, as
-returned by tell().  The current file position is unchanged.
-Returns the new absolute position.
+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_StringIO_truncate_impl(stringio *self, PyObject *arg)
-/*[clinic end generated code: output=6072439c2b01d306 input=748619a494ba53ad]*/
+/*[clinic end generated code: output=6072439c2b01d306 input=0874f292f43baecb]*/
 {
     Py_ssize_t size;