diff --git a/mtar/README.md b/mtar/README.md
index 53cb015..23fd360 100644
--- a/mtar/README.md
+++ b/mtar/README.md
@@ -4,7 +4,18 @@ mtar is a file format for storing multiple files in a single file, much like tar
 
 ## File format
 
-mtar archive files consist of a number of file entries, in the following format:
+mtar archive files consist of a number of file entries, in any combination of the following formats:
+
+### v1 format
+
+- 0xFFFF (for differentiating versioned headers from legacy v0 headers)
+- version byte, with the value 1
+- 2 bytes file name length
+- the file name
+- 8 bytes file content length
+- the file contents
+
+### Legacy v0 format
 
 - 2 bytes file name length
 - the file name