diff --git a/README.md b/README.md
index 3b1a1b0..74baaad 100644
--- a/README.md
+++ b/README.md
@@ -78,24 +78,28 @@ relative to the beginning of the file.
 ```
 const File = struct {
     magic: [4]u8,
-    version: u3,
-    shells_oob: u1,
+    version: u4,
     padding: u4,
+    num_shells: u8,
     num_users: u32,
     num_groups: u32,
-    <... TBD ...>
+    offset_cmph_gid2group: u26,
+    offset_cmph_uid2user: u26,
+    offset_cmph_groupname2group: u26,
+    offset_cmph_username2user: u26,
+    offset_sorted_groups: u26,
+    offset_sorted_users: u26,
+    offset_groupmembers: u26,
+    offset_additional_gids: u26,
 }
 ```
 
-`magic` must be 0xf09fa4b7, and `version` must be `0`. The remaining fields are
-indices to further sections of the file with their sizes in bytes. All numbers
-are little-endian.
+`magic` is 0xf09fa4b7, and `version` must be `0`. Offsets are indices to
+further sections of the file, with zero being the first block (the magic
+number). As all blobs are 64-byte aligned, the offsets are pointing to the
+beginning of the 64-byte "block" (thus u26). All numbers are little-endian.
 
-What's remaining, variable-length:
-
-1. A lookup list of shells (if `shells_oob` is True).
-2. 4 indices mentioned above.
-3. <...>
+As of writing the file header is 40 bytes.
 
 Primitive types:
 
@@ -116,8 +120,9 @@ const User = struct {
     gid: u32,
     // pointer to a separate structure that contains a list of gids
     additional_gids_offset: u29,
-    padding: u1,
-    shell_len: u6,
+    // shell is a different story, documented elsewhere.
+    shell_here: u1,
+    shell_len_or_place: u6,
     home_len: u6,
     username_len: u6,
     gecos_len: u8,
@@ -127,5 +132,10 @@ const User = struct {
 }
 ```
 
+TODO explain:
+- shells
+- `additional_gids`
+- `members`
+
 [git-subtrac]: https://github.com/apenwarr/git-subtrac/
 [cmph]: http://cmph.sourceforge.net/