diff --git a/src/include/mx/api/OttavaData.h b/src/include/mx/api/OttavaData.h
index bc1f1736f..f28e59629 100644
--- a/src/include/mx/api/OttavaData.h
+++ b/src/include/mx/api/OttavaData.h
@@ -28,7 +28,14 @@ class OttavaStart
     SpannerStart spannerStart;
     OttavaType ottavaType;
 
-    OttavaStart() : spannerStart{}, ottavaType{OttavaType::unspecified}
+    // Octave-shift size fidelity knob. The size value (8 or 15) follows from ottavaType, so a
+    // 15ma/15mb line always writes size="15" while an 8va/8vb line omits the redundant, spec-
+    // default size="8". When true, the writer also emits that redundant size="8"; the reader sets
+    // it when the source spelled the attribute out. Leave false (the default) when authoring. It
+    // has no effect on a 15ma/15mb line, whose size is always written.
+    bool writeDefaultSize;
+
+    OttavaStart() : spannerStart{}, ottavaType{OttavaType::unspecified}, writeDefaultSize{false}
     {
     }
 };
@@ -52,6 +59,7 @@ class OttavaStop
 MXAPI_EQUALS_BEGIN(OttavaStart)
 MXAPI_EQUALS_MEMBER(spannerStart)
 MXAPI_EQUALS_MEMBER(ottavaType)
+MXAPI_EQUALS_MEMBER(writeDefaultSize)
 MXAPI_EQUALS_END;
 MXAPI_NOT_EQUALS_AND_VECTORS(OttavaStart);
 
diff --git a/src/private/mx/impl/DirectionReader.cpp b/src/private/mx/impl/DirectionReader.cpp
index b62cc8753..528c112ab 100644
--- a/src/private/mx/impl/DirectionReader.cpp
+++ b/src/private/mx/impl/DirectionReader.cpp
@@ -840,6 +840,8 @@ void DirectionReader::parseOctaveShift(const core::DirectionType &directionType)
     api::OttavaStart start;
     start.spannerStart = impl::getSpannerStart(octaveShift);
     start.ottavaType = ottavaType;
+    const bool isEightLine = ottavaType == api::OttavaType::o8va || ottavaType == api::OttavaType::o8vb;
+    start.writeDefaultSize = isEightLine && octaveShift.size().has_value();
     start.spannerStart.tickTimePosition = myCursor.tickTimePosition;
     myOutDirectionData.ottavaStarts.emplace_back(std::move(start));
     appendOrderedComponent(api::DirectionComponentKind::ottavaStart,
diff --git a/src/private/mx/impl/DirectionWriter.cpp b/src/private/mx/impl/DirectionWriter.cpp
index d295121ee..38bb91838 100644
--- a/src/private/mx/impl/DirectionWriter.cpp
+++ b/src/private/mx/impl/DirectionWriter.cpp
@@ -399,32 +399,42 @@ void DirectionWriter::emitOttavaStart(const api::OttavaStart &ottavaStart, core:
         os.setNumber(core::NumberLevel{*number});
     }
 
+    int sizeValue = 8;
+
     switch (ottavaStart.ottavaType)
     {
     case api::OttavaType::o15ma: {
         os.setType(core::UpDownStopContinue::down());
-        os.setSize(15);
+        sizeValue = 15;
         break;
     }
     case api::OttavaType::o15mb: {
         os.setType(core::UpDownStopContinue::up());
-        os.setSize(15);
+        sizeValue = 15;
         break;
     }
     case api::OttavaType::o8va: {
         os.setType(core::UpDownStopContinue::down());
-        os.setSize(8);
+        sizeValue = 8;
         break;
     }
     case api::OttavaType::o8vb: {
         os.setType(core::UpDownStopContinue::up());
-        os.setSize(8);
+        sizeValue = 8;
         break;
     }
     default:
         break;
     }
 
+    // size follows from ottavaType; a 15ma/15mb line's size encodes the two-octave shift, so it is
+    // always written, while the redundant default size="8" is emitted only when writeDefaultSize is
+    // set (the source spelled it out).
+    if (sizeValue != 8 || ottavaStart.writeDefaultSize)
+    {
+        os.setSize(sizeValue);
+    }
+
     core::DirectionType dt{};
     dt.setChoice(core::DirectionTypeChoice::octaveShift(os));
     addDirectionType(std::move(dt), direction);