diff --git a/README.md b/README.md
index d46b494..8c23154 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,7 @@ The format string shall be (in Python syntax) `r'<i([^-]+)-([^-]*)-([0-9A-Fa-f]+
 
 The first capture group shall be a format. The currently recognized formats are `gb` and `raw`, though any format can be used. A program SHOULD gracefully report and exit on an unknown or unsupported format.  The identifier of a format may not include a hyphen (dash).
 
-The second capture group (may be empty) is defined by the format.
+The second capture group (may be empty) shall be known as the header and its syntax shall be defined by the format.
 
 The third capture group contains the image data and is defined by the format.
 
@@ -22,7 +22,7 @@ While there is no theoretical limit to the size of the image you can send, you a
 
 For an explanation of the format, see [here](http://www.huderlem.com/demos/gameboy2bpp.html).
 
-The second capture group specifies the width and height in 8x8 tiles.
+The header has this syntax: `r'(\d+).(\d+)'`.  The first and second capture groups specify the width and height in 8x8 tiles.
 
 The third capture group 
 
@@ -30,10 +30,10 @@ Example line: `<igb-1.1-FF007EFF858189839385A58BC9977EFF>`
 Results in:  
 ![Pokemon Red window](https://ttm.sh/qX.png)
 
-### `raw` - Raw 2-shade monochrome
+### `raw` - Raw arbitrary bitmaps
 
-2 shade monochrome
+8x8, 2 shade monochrome
 
-Example line: `<iraw-8.8-FF818181818181FF>`  
+Example line: `<iraw-8.8.1-FF818181818181FF>`  
 Results in:  
 ![box](https://ttm.sh/qy.png)
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