diff --git a/src/include/mx/api/ClefData.h b/src/include/mx/api/ClefData.h
index 2f0fd89c6..c4feba6cd 100644
--- a/src/include/mx/api/ClefData.h
+++ b/src/include/mx/api/ClefData.h
@@ -21,6 +21,11 @@ enum class ClefSymbol
     jianpu
 };
 
+// Where a clef sits relative to its measure. A clef at the start of a measure normally prints
+// before the barline; beforeBarline and afterBarline choose between the two, which matters for cue
+// clefs and for the clef that resumes after a repeated section. midMeasure describes a clef change
+// partway through the measure, positioned by tickTimePosition. unspecified leaves the choice to the
+// notation program.
 enum class ClefLocation
 {
     unspecified,
@@ -53,6 +58,12 @@ class ClefData
     bool isOctaveChangeSpecified;
     int tickTimePosition;
     ClefLocation location;
+    // Marks a supplementary clef placed on the staff alongside the regular one - a cue clef, or a
+    // second clef in effect at the same time. MusicXML writes this as clef@additional="yes"; such a
+    // clef sits at a non-standard line position, is not restated at the start of each system, and
+    // notation software disregards its line. unspecified (the default) omits the attribute, which
+    // describes an ordinary clef.
+    Bool additional;
     // Visibility of the clef via the MusicXML print-object attribute.
     // unspecified -> omit the attribute, yes/no -> write print-object verbatim.
     Bool printObject;
@@ -86,6 +97,7 @@ MXAPI_EQUALS_MEMBER(octaveChange)
 MXAPI_EQUALS_MEMBER(isOctaveChangeSpecified)
 MXAPI_EQUALS_MEMBER(tickTimePosition)
 MXAPI_EQUALS_MEMBER(location)
+MXAPI_EQUALS_MEMBER(additional)
 MXAPI_EQUALS_MEMBER(printObject)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(ClefData);
diff --git a/src/private/mx/api/ClefData.cpp b/src/private/mx/api/ClefData.cpp
index 4219b7dc6..ccd1e103f 100644
--- a/src/private/mx/api/ClefData.cpp
+++ b/src/private/mx/api/ClefData.cpp
@@ -12,7 +12,7 @@ namespace api
 ClefData::ClefData()
     : writeStaffNumber{Bool::unspecified}, symbol{DEFAULT_CLEF_SYMBOL}, line{DEFAULT_CLEF_LINE}, isLineSpecified{true},
       octaveChange{DEFAULT_CLEF_OCTAVE_CHANGE}, isOctaveChangeSpecified{true}, tickTimePosition{0},
-      location{ClefLocation::unspecified}, printObject{Bool::unspecified}
+      location{ClefLocation::unspecified}, additional{Bool::unspecified}, printObject{Bool::unspecified}
 {
 }
 
diff --git a/src/private/mx/impl/MeasureReader.cpp b/src/private/mx/impl/MeasureReader.cpp
index f6fa4f001..ff689f064 100644
--- a/src/private/mx/impl/MeasureReader.cpp
+++ b/src/private/mx/impl/MeasureReader.cpp
@@ -1114,6 +1114,11 @@ void MeasureReader::importClef(const core::Clef &inClef) const
         clefData.isOctaveChangeSpecified = false;
     }
 
+    if (inClef.additional().has_value())
+    {
+        clefData.additional = converter.convert(*inClef.additional());
+    }
+
     if (inClef.printObject().has_value())
     {
         clefData.printObject = converter.convert(*inClef.printObject());
diff --git a/src/private/mx/impl/PropertiesWriter.cpp b/src/private/mx/impl/PropertiesWriter.cpp
index fdf165197..bd1ee6894 100644
--- a/src/private/mx/impl/PropertiesWriter.cpp
+++ b/src/private/mx/impl/PropertiesWriter.cpp
@@ -354,6 +354,22 @@ void PropertiesWriter::writeClef(int staffIndex, const api::ClefData &inClefData
         cg.setClefOctaveChange(inClefData.octaveChange);
     }
 
+    if (inClefData.additional != api::Bool::unspecified)
+    {
+        mxClef.setAdditional(converter.convert(inClefData.additional));
+    }
+
+    // after-barline states which side of the barline a start-of-measure clef prints on. MusicXML
+    // ignores it on mid-measure clefs, so those emit nothing.
+    if (inClefData.location == api::ClefLocation::afterBarline)
+    {
+        mxClef.setAfterBarline(core::YesNo::yes());
+    }
+    else if (inClefData.location == api::ClefLocation::beforeBarline)
+    {
+        mxClef.setAfterBarline(core::YesNo::no());
+    }
+
     if (inClefData.printObject != api::Bool::unspecified)
     {
         mxClef.setPrintObject(converter.convert(inClefData.printObject));
diff --git a/src/private/mxtest/api/ClefDataTest.cpp b/src/private/mxtest/api/ClefDataTest.cpp
index 38e7f069c..b2dba1316 100644
--- a/src/private/mxtest/api/ClefDataTest.cpp
+++ b/src/private/mxtest/api/ClefDataTest.cpp
@@ -31,6 +31,25 @@ ScoreData makeScoreWithClef(Bool printObject)
     staff.voices[0].notes.emplace_back();
     return score;
 }
+
+ScoreData makeScoreWithAdditionalClef(Bool additional)
+{
+    ScoreData score = makeScoreWithClef(Bool::unspecified);
+    score.parts.front().measures.front().staves.front().clefs.front().additional = additional;
+    return score;
+}
+
+ScoreData makeScoreWithClefLocation(ClefLocation location)
+{
+    ScoreData score = makeScoreWithClef(Bool::unspecified);
+    score.parts.front().measures.front().staves.front().clefs.front().location = location;
+    return score;
+}
+
+const ClefData &firstClef(const ScoreData &score)
+{
+    return score.parts.front().measures.front().staves.front().clefs.front();
+}
 } // namespace mxtest::api::clef_data_test
 
 TEST(clefPrintObjectNoRoundTrip, ClefData)
@@ -78,4 +97,156 @@ TEST(clefPrintObjectUnspecifiedIsOmitted, ClefData)
 
 T_END;
 
+TEST(clefAdditionalYesRoundTrip, ClefData)
+{
+    const auto xml = mxtest::toXml(mxtest::api::clef_data_test::makeScoreWithAdditionalClef(Bool::yes));
+    CHECK(xml.find("additional=\"yes\"") != std::string::npos);
+
+    const auto out = mxtest::fromXml(xml);
+    CHECK_EQUAL(1, out.parts.size());
+    CHECK_EQUAL(1, out.parts.front().measures.size());
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.size());
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.front().clefs.size());
+    CHECK(Bool::yes == out.parts.front().measures.front().staves.front().clefs.front().additional);
+}
+
+T_END;
+
+TEST(clefAdditionalNoRoundTrip, ClefData)
+{
+    const auto xml = mxtest::toXml(mxtest::api::clef_data_test::makeScoreWithAdditionalClef(Bool::no));
+    CHECK(xml.find("additional=\"no\"") != std::string::npos);
+
+    const auto out = mxtest::fromXml(xml);
+    CHECK_EQUAL(1, out.parts.size());
+    CHECK_EQUAL(1, out.parts.front().measures.size());
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.size());
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.front().clefs.size());
+    CHECK(Bool::no == out.parts.front().measures.front().staves.front().clefs.front().additional);
+}
+
+T_END;
+
+TEST(clefAdditionalUnspecifiedIsOmitted, ClefData)
+{
+    const auto xml = mxtest::toXml(mxtest::api::clef_data_test::makeScoreWithAdditionalClef(Bool::unspecified));
+    CHECK(xml.find("additional=") == std::string::npos);
+
+    const auto out = mxtest::fromXml(xml);
+    CHECK_EQUAL(1, out.parts.size());
+    CHECK_EQUAL(1, out.parts.front().measures.size());
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.size());
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.front().clefs.size());
+    CHECK(Bool::unspecified == out.parts.front().measures.front().staves.front().clefs.front().additional);
+}
+
+T_END;
+
+TEST(clefAfterBarlineYesRoundTrip, ClefData)
+{
+    using namespace mxtest::api::clef_data_test;
+    const auto xml = mxtest::toXml(makeScoreWithClefLocation(ClefLocation::afterBarline));
+    CHECK(xml.find("after-barline=\"yes\"") != std::string::npos);
+
+    const auto out = mxtest::fromXml(xml);
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.front().clefs.size());
+    CHECK(ClefLocation::afterBarline == firstClef(out).location);
+}
+
+T_END;
+
+TEST(clefAfterBarlineNoRoundTrip, ClefData)
+{
+    using namespace mxtest::api::clef_data_test;
+    const auto xml = mxtest::toXml(makeScoreWithClefLocation(ClefLocation::beforeBarline));
+    CHECK(xml.find("after-barline=\"no\"") != std::string::npos);
+
+    const auto out = mxtest::fromXml(xml);
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.front().clefs.size());
+    CHECK(ClefLocation::beforeBarline == firstClef(out).location);
+}
+
+T_END;
+
+TEST(clefAfterBarlineUnspecifiedIsOmitted, ClefData)
+{
+    using namespace mxtest::api::clef_data_test;
+    const auto xml = mxtest::toXml(makeScoreWithClefLocation(ClefLocation::unspecified));
+    CHECK(xml.find("after-barline=") == std::string::npos);
+
+    const auto out = mxtest::fromXml(xml);
+    CHECK_EQUAL(1, out.parts.front().measures.front().staves.front().clefs.size());
+    CHECK(ClefLocation::unspecified == firstClef(out).location);
+}
+
+T_END;
+
+TEST(clefAllFieldsSurviveWrite, ClefData)
+{
+    using namespace mxtest::api::clef_data_test;
+    ScoreData score = makeScoreWithClef(Bool::unspecified);
+    auto &clef = score.parts.front().measures.front().staves.front().clefs.front();
+    clef.writeStaffNumber = Bool::yes;
+    clef.symbol = ClefSymbol::f;
+    clef.line = 4;
+    clef.isLineSpecified = true;
+    clef.octaveChange = -1;
+    clef.isOctaveChangeSpecified = true;
+    clef.tickTimePosition = 0;
+    clef.location = ClefLocation::afterBarline;
+    clef.additional = Bool::yes;
+    clef.printObject = Bool::no;
+    const ClefData expected = clef;
+
+    const auto out = mxtest::fromXml(mxtest::toXml(score));
+    const auto &actual = firstClef(out);
+    CHECK(expected.writeStaffNumber == actual.writeStaffNumber);
+    CHECK(expected.symbol == actual.symbol);
+    CHECK_EQUAL(expected.line, actual.line);
+    CHECK_EQUAL(expected.isLineSpecified, actual.isLineSpecified);
+    CHECK_EQUAL(expected.octaveChange, actual.octaveChange);
+    CHECK_EQUAL(expected.isOctaveChangeSpecified, actual.isOctaveChangeSpecified);
+    CHECK_EQUAL(expected.tickTimePosition, actual.tickTimePosition);
+    CHECK(expected.location == actual.location);
+    CHECK(expected.additional == actual.additional);
+    CHECK(expected.printObject == actual.printObject);
+    CHECK(expected == actual);
+}
+
+T_END;
+
+TEST(clefUnspecifiedElementsSurviveWrite, ClefData)
+{
+    using namespace mxtest::api::clef_data_test;
+    ScoreData score = makeScoreWithClef(Bool::unspecified);
+    auto &clef = score.parts.front().measures.front().staves.front().clefs.front();
+    clef.symbol = ClefSymbol::c;
+    clef.isLineSpecified = false;
+    clef.line = 3;
+    clef.isOctaveChangeSpecified = false;
+    clef.octaveChange = 0;
+    clef.writeStaffNumber = Bool::no;
+    const ClefData expected = clef;
+
+    const auto xml = mxtest::toXml(score);
+    CHECK(xml.find("<line>") == std::string::npos);
+    CHECK(xml.find("<clef-octave-change>") == std::string::npos);
+    CHECK(xml.find("<clef>") != std::string::npos);
+
+    const auto out = mxtest::fromXml(xml);
+    const auto &actual = firstClef(out);
+    CHECK(expected.symbol == actual.symbol);
+    CHECK_EQUAL(expected.line, actual.line);
+    CHECK_EQUAL(expected.isLineSpecified, actual.isLineSpecified);
+    CHECK_EQUAL(expected.octaveChange, actual.octaveChange);
+    CHECK_EQUAL(expected.isOctaveChangeSpecified, actual.isOctaveChangeSpecified);
+
+    // writeStaffNumber records only a divergence from the automatic rule. On a single-staff part
+    // the automatic rule already omits the number, so no agrees with it and reads back as
+    // unspecified. The emitted XML is the same either way.
+    CHECK(Bool::unspecified == actual.writeStaffNumber);
+}
+
+T_END;
+
 #endif