diff --git a/man/groff.7.man b/man/groff.7.man
index fc1c958d..bb244ab3 100644
--- a/man/groff.7.man
+++ b/man/groff.7.man
@@ -3885,6 +3885,13 @@ Current input line number.
 .REG .C
 1\~if compatibility mode is in effect, 0\~otherwise.
 .
+Always\~0 in a
+.request .do
+request;
+see
+.register .cp
+below.
+.
 .TPx
 .REG .cdp
 The depth of the last glyph added to the current environment.
@@ -3908,6 +3915,15 @@ It is positive if the glyph extends above the baseline.
 1\~if colors are enabled, 0\~otherwise.
 .
 .TPx
+.REG .cp
+Within a
+.request .do
+request,
+holds the saved value of compatibilty mode (see
+.register .C
+above).
+.
+.TPx
 .REG .csk
 The skew of the last glyph added to the current environment.
 .
diff --git a/man/groff_diff.7.man b/man/groff_diff.7.man
index 35658306..d406b790 100644
--- a/man/groff_diff.7.man
+++ b/man/groff_diff.7.man
@@ -2760,6 +2760,15 @@ It is positive if the glyph extends above the baseline.
 1\~if colors are enabled, 0\~otherwise.
 .
 .TP
+.B \[rs]n[.cp]
+Within a
+.B .do
+request,
+holds the saved value of compatibilty mode (see
+.B \[rs]n[.C]
+above).
+.
+.TP
 .B \[rs]n[.csk]
 The skew of the last glyph added to the current environment.
 .
@@ -3804,6 +3813,104 @@ escape sequence can be helpful in avoiding these escape sequences in
 names.
 .
 .P
+The register
+.B \[rs]n[.cp]
+is specialized and may require a statement of rationale.
+.
+When writing your own macro packages or documents that use
+.I groff
+features and will be sourced by others with the
+.B .so
+request,
+you may desire correct operation regardless of compatibility mode in the
+surrounding context.
+.
+It may occur to you to save the existing value of
+.B \[rs]n(.C
+into a temporary register,
+say,
+.BR _C ,
+at the beginning of your file,
+turn compatibility mode off with
+.BR .cp\~0 ,
+then restore it from that register at the end with the
+.B .cp
+request.
+.
+At the same time,
+a modular design may lead you to multiple layers of inclusion.
+.
+You cannot use the same register name everywhere or you risk
+\[lq]clobbering\[rq] the value from an outer scope.
+.
+The two-character register namespace of AT&T
+.I troff
+is confining and mnemonically difficult;
+you may wish to use
+.IR groff 's
+more capacious namespace.
+.
+However,
+attempting
+.RS
+.RS
+.EX
+\&.nr _my_saved_C \[rs]n(.C
+.EE
+.RE
+.RE
+will not work in compatibility mode;
+the register name is too long.
+.
+\[lq]This is exactly what
+.B .do
+is for,\[rq] you think.
+.RS
+.RS
+.EX
+\&.do nr _my_saved_C \[rs]n(.C
+.EE
+.RE
+.RE
+.
+The foregoing will always save zero to your register,
+because
+.B .do
+turns compatibility mode off.
+.
+It will not serve to have
+.B .do
+report the saved compatibility mode in
+.B \[rs]n(.C
+for this special case,
+because the request can do anything
+(except change compatibility mode in its enclosing scope),
+including call macros and source files,
+and the value of
+.B \[rs]n(.C
+must be reliable there.
+.
+What you need is:
+.RS
+.RS
+.EX
+\&.do nr _my_saved_C \[rs]n[.cp]
+\&.cp 0
+.EE
+.RE
+.RE
+at the beginning of your file,
+followed by
+.RS
+.RS
+.EX
+\&.cp _my_saved_C
+.EE
+.RE
+.RE
+at the end.
+.
+.P
 Fractional point sizes cause one noteworthy incompatibility.
 .
 In
diff --git a/src/roff/groff/groff.am b/src/roff/groff/groff.am
index a7999dc0..86f7acd5 100644
--- a/src/roff/groff/groff.am
+++ b/src/roff/groff/groff.am
@@ -41,6 +41,7 @@ groffoptsdir = $(libprogramdir)
 groffopts_DATA = $(GROFF_OPTS_OUTPUT)
 
 groff_TESTS = \
+  src/roff/groff/tests/dot-cp_register_works.sh \
   src/roff/groff/tests/on_latin1_device_oq_is_0x27.sh \
   src/roff/groff/tests/recognize_end_of_sentence.sh \
   src/roff/groff/tests/regression_savannah_56555.sh \
diff --git a/src/roff/groff/tests/dot-cp_register_works.sh b/src/roff/groff/tests/dot-cp_register_works.sh
new file mode 100755
index 00000000..2e3fee19
--- /dev/null
+++ b/src/roff/groff/tests/dot-cp_register_works.sh
@@ -0,0 +1,48 @@
+#!/bin/sh
+#
+# Copyright (C) 2020 Free Software Foundation, Inc.
+#
+# This file is part of groff.
+#
+# groff is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# groff is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+# for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+groff="${abs_top_builddir:-.}/test-groff"
+
+DOC='.pl 1v
+A
+.do if 1 \n[.cp] \" Get initial compatibility state (depends on -C).
+B
+.do if 1 \n[.cp] \" Did observing the state change it?
+.cp 1
+C
+.do if 1 \n[.cp] \" Saved compatibility state should be 1 now.
+.cp 0
+D
+.do if 1 \n[.cp] \" Verify 1->0 transition.
+.cp 1
+E
+.do if 1 \n[.cp] \" Verify 0->1 transition.
+.cp 0
+F
+.if !\n[.C] \n[.cp] \" Outside of .do context, should return -1.
+'
+
+set -e
+
+printf "%s" "$DOC" | "$groff" -Tascii \
+    | grep -x "A 0 B 0 C 1 D 0 E 1 F -1"
+
+printf "%s" "$DOC" | "$groff" -C -Tascii \
+    | grep -x "A 1 B 1 C 1 D 0 E 1 F -1"
diff --git a/src/roff/troff/input.cpp b/src/roff/troff/input.cpp
index 26394ade..7d3b230c 100644
--- a/src/roff/troff/input.cpp
+++ b/src/roff/troff/input.cpp
@@ -103,6 +103,7 @@ static symbol end_macro_name;
 static symbol blank_line_macro_name;
 static symbol leading_spaces_macro_name;
 static int compatible_flag = 0;
+static int do_old_compatible_flag = -1;	// for .do request
 int ascii_output_flag = 0;
 int suppress_output_flag = 0;
 int is_html = 0;
@@ -2640,14 +2641,16 @@ static void trapping_blank_line()
 
 void do_request()
 {
-  int old_compatible_flag = compatible_flag;
+  assert(do_old_compatible_flag == -1);
+  do_old_compatible_flag = compatible_flag;
   compatible_flag = 0;
   symbol nm = get_name();
   if (nm.is_null())
     skip_line();
   else
     interpolate_macro(nm, 1);
-  compatible_flag = old_compatible_flag;
+  compatible_flag = do_old_compatible_flag;
+  do_old_compatible_flag = -1;
   request_or_macro *p = lookup_request(nm);
   macro *m = p->to_macro();
   if (m)
@@ -8297,6 +8300,7 @@ void init_input_requests()
   number_reg_dictionary.define(".$", new nargs_reg);
   number_reg_dictionary.define(".br", new break_flag_reg);
   number_reg_dictionary.define(".C", new constant_int_reg(&compatible_flag));
+  number_reg_dictionary.define(".cp", new constant_int_reg(&do_old_compatible_flag));
   number_reg_dictionary.define(".O", new variable_reg(&begin_level));
   number_reg_dictionary.define(".c", new lineno_reg);
   number_reg_dictionary.define(".color", new constant_int_reg(&color_flag));