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diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index 70a99fd..cab5bdd 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -252,3 +252,80 @@ Pack file entry: <+
     corresponding packfile.
 
     20-byte SHA-1-checksum of all of the above.
+
+== multi-pack-index (MIDX) files have the following format:
+
+The multi-pack-index files refer to multiple pack-files and loose objects.
+
+In order to allow extensions that add extra data to the MIDX, we organize
+the body into "chunks" and provide a lookup table at the beginning of the
+body. The header includes certain length values, such as the number of packs,
+the number of base MIDX files, hash lengths and types.
+
+All 4-byte numbers are in network order.
+
+HEADER:
+
+	4-byte signature:
+	    The signature is: {'M', 'I', 'D', 'X'}
+
+	1-byte version number:
+	    Git only writes or recognizes version 1.
+
+	1-byte Object Id Version
+	    Git only writes or recognizes version 1 (SHA1).
+
+	1-byte number of "chunks"
+
+	1-byte number of base multi-pack-index files:
+	    This value is currently always zero.
+
+	4-byte number of pack files
+
+CHUNK LOOKUP:
+
+	(C + 1) * 12 bytes providing the chunk offsets:
+	    First 4 bytes describe chunk id. Value 0 is a terminating label.
+	    Other 8 bytes provide offset in current file for chunk to start.
+	    (Chunks are provided in file-order, so you can infer the length
+	    using the next chunk position if necessary.)
+
+	The remaining data in the body is described one chunk at a time, and
+	these chunks may be given in any order. Chunks are required unless
+	otherwise specified.
+
+CHUNK DATA:
+
+	Packfile Names (ID: {'P', 'N', 'A', 'M'})
+	    Stores the packfile names as concatenated, null-terminated strings.
+	    Packfiles must be listed in lexicographic order for fast lookups by
+	    name. This is the only chunk not guaranteed to be a multiple of four
+	    bytes in length, so should be the last chunk for alignment reasons.
+
+	OID Fanout (ID: {'O', 'I', 'D', 'F'})
+	    The ith entry, F[i], stores the number of OIDs with first
+	    byte at most i. Thus F[255] stores the total
+	    number of objects.
+
+	OID Lookup (ID: {'O', 'I', 'D', 'L'})
+	    The OIDs for all objects in the MIDX are stored in lexicographic
+	    order in this chunk.
+
+	Object Offsets (ID: {'O', 'O', 'F', 'F'})
+	    Stores two 4-byte values for every object.
+	    1: The pack-int-id for the pack storing this object.
+	    2: The offset within the pack.
+		If all offsets are less than 2^31, then the large offset chunk
+		will not exist and offsets are stored as in IDX v1.
+		If there is at least one offset value larger than 2^32-1, then
+		the large offset chunk must exist. If the large offset chunk
+		exists and the 31st bit is on, then removing that bit reveals
+		the row in the large offsets containing the 8-byte offset of
+		this object.
+
+	[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
+	    8-byte offsets into large packfiles.
+
+TRAILER:
+
+	20-byte SHA1-checksum of the above contents.