diff --git a/main.hpp b/main.hpp
index 6b979cfa..5f7128b5 100644
--- a/main.hpp
+++ b/main.hpp
@@ -13,10 +13,11 @@
 struct index {
 	long long start = 0;  // offset of data in geom file
 	long long end = 0;
-	unsigned long long ix = 0;		// quadkey or hilbert index
-	unsigned short t : 2;			// geometry type
-	unsigned short dup : 8;			// instance number of identical geometry
-	unsigned long long seq : (64 - 2 - 8);	// input sequence number; pack with t and dup to stay in 32 bytes
+	unsigned long long ix = 0;		     // quadkey or hilbert index
+	unsigned short segment;			     // which reader thread produced this feature
+	unsigned short t : 2;			     // geometry type
+	unsigned short dup : 8;			     // instance number of identical geometry
+	unsigned long long seq : (64 - 16 - 2 - 8);  // input sequence number; pack with segment and t and dup to stay in 32 bytes
 
 	index()
 	    : t(0),
diff --git a/serial.cpp b/serial.cpp
index 224cd886..ef1ed435 100644
--- a/serial.cpp
+++ b/serial.cpp
@@ -883,6 +883,7 @@ int serialize_feature(struct serialization_state *sst, serial_feature &sf, std::
 	index.start = geomstart;
 	index.end = r->geompos;
 	index.seq = *(sst->layer_seq);
+	index.segment = sst->segment;
 	index.t = sf.t;
 	index.ix = bbox_index;