diff --git a/lucene/core/src/java/org/apache/lucene/codecs/blocktree/BlockTreeTermsReader.java b/lucene/core/src/java/org/apache/lucene/codecs/blocktree/BlockTreeTermsReader.java
index c1d43eb..e01288e 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/blocktree/BlockTreeTermsReader.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/blocktree/BlockTreeTermsReader.java
@@ -84,7 +84,7 @@
 
   /** Extension of terms file */
   static final String TERMS_EXTENSION = "tim";
-  final static String TERMS_CODEC_NAME = "BLOCK_TREE_TERMS_DICT";
+  final static String TERMS_CODEC_NAME = "BlockTreeTermsDict";
 
   /** Initial terms format. */
   public static final int VERSION_START = 0;
@@ -94,7 +94,7 @@
 
   /** Extension of terms index file */
   static final String TERMS_INDEX_EXTENSION = "tip";
-  final static String TERMS_INDEX_CODEC_NAME = "BLOCK_TREE_TERMS_INDEX";
+  final static String TERMS_INDEX_CODEC_NAME = "BlockTreeTermsIndex";
 
   // Open input to the main terms dict file (_X.tib)
   final IndexInput termsIn;
diff --git a/lucene/core/src/java/org/apache/lucene/codecs/lucene50/Lucene50DocValuesFormat.java b/lucene/core/src/java/org/apache/lucene/codecs/lucene50/Lucene50DocValuesFormat.java
index 7d23482..c580805 100644
--- a/lucene/core/src/java/org/apache/lucene/codecs/lucene50/Lucene50DocValuesFormat.java
+++ b/lucene/core/src/java/org/apache/lucene/codecs/lucene50/Lucene50DocValuesFormat.java
@@ -184,7 +184,7 @@
   
   static final String DATA_CODEC = "Lucene50DocValuesData";
   static final String DATA_EXTENSION = "dvd";
-  static final String META_CODEC = "Lucene50ValuesMetadata";
+  static final String META_CODEC = "Lucene50DocValuesMetadata";
   static final String META_EXTENSION = "dvm";
   static final int VERSION_START = 0;
   static final int VERSION_CURRENT = VERSION_START;
diff --git a/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java b/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java
index cf81147..2210cbf 100644
--- a/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java
+++ b/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java
@@ -60,7 +60,7 @@
  * Files:
  * <ul>
  * <li><tt>segments_N</tt>: Header, Version, NameCounter, SegCount, &lt;SegName,
- * SegID, SegCodec, DelGen, DeletionCount, FieldInfosGen, DocValuesGen,
+ * HasSegID, SegID, SegCodec, DelGen, DeletionCount, FieldInfosGen, DocValuesGen,
  * UpdatesFiles&gt;<sup>SegCount</sup>, CommitUserData, Footer
  * </ul>
  * </p>
@@ -72,6 +72,8 @@
  * {@link DataOutput#writeInt Int32}</li>
  * <li>Generation, Version, DelGen, Checksum, FieldInfosGen, DocValuesGen --&gt;
  * {@link DataOutput#writeLong Int64}</li>
+ * <li>HasSegID --&gt; {@link DataOutput#writeByte Int8}</li>
+ * <li>SegID --&gt; {@link DataOutput#writeByte Int8<sup>ID_LENGTH</sup>}</li>
  * <li>SegName, SegCodec --&gt; {@link DataOutput#writeString String}</li>
  * <li>CommitUserData --&gt; {@link DataOutput#writeStringStringMap
  * Map&lt;String,String&gt;}</li>
@@ -94,7 +96,10 @@
  * <li>DeletionCount records the number of deleted documents in this segment.</li>
  * <li>SegCodec is the {@link Codec#getName() name} of the Codec that encoded
  * this segment.</li>
- * <li>SegID is the id of the Codec that encoded this segment. </li>
+ * <li>HasSegID is nonzero if the segment has an identifier. Otherwise, when it is 0
+ * the identifier is {@code null} and no SegID is written. Null only happens for Lucene
+ * 4.x segments referenced in commits.</li>
+ * <li>SegID is the identifier of the Codec that encoded this segment. </li>
  * <li>CommitUserData stores an optional user-supplied opaque
  * Map&lt;String,String&gt; that was passed to
  * {@link IndexWriter#setCommitData(java.util.Map)}.</li>
diff --git a/lucene/sandbox/src/java/org/apache/lucene/codecs/idversion/VersionBlockTreeTermsWriter.java b/lucene/sandbox/src/java/org/apache/lucene/codecs/idversion/VersionBlockTreeTermsWriter.java
index e9971b7..a055396 100644
--- a/lucene/sandbox/src/java/org/apache/lucene/codecs/idversion/VersionBlockTreeTermsWriter.java
+++ b/lucene/sandbox/src/java/org/apache/lucene/codecs/idversion/VersionBlockTreeTermsWriter.java
@@ -118,7 +118,7 @@
 
   /** Extension of terms file */
   static final String TERMS_EXTENSION = "tiv";
-  final static String TERMS_CODEC_NAME = "VERSION_BLOCK_TREE_TERMS_DICT";
+  final static String TERMS_CODEC_NAME = "VersionBlockTreeTermsDict";
 
   /** Initial terms format. */
   public static final int VERSION_START = 1;
@@ -128,7 +128,7 @@
 
   /** Extension of terms index file */
   static final String TERMS_INDEX_EXTENSION = "tipv";
-  final static String TERMS_INDEX_CODEC_NAME = "VERSION_BLOCK_TREE_TERMS_INDEX";
+  final static String TERMS_INDEX_CODEC_NAME = "VersionBlockTreeTermsIndex";
 
   private final IndexOutput out;
   private final IndexOutput indexOut;