diff --git a/Extensions.md b/Extensions.md
index 5128963..2fe9ee4 100644
--- a/Extensions.md
+++ b/Extensions.md
@@ -15,16 +15,18 @@ None known yet...
 
 Tahoe URIs may refer to either immutable or mutable content.  URIs for immutable data are simple to cache by mapping the URI to the contents.
 
-Mutability is independent of read-only capabilities.  A read-only capability may refer to mutable content.
+Mutability is independent of read-only capabilities.  A read-only capability may refer to mutable content (i.e. *you* can't write to it, but somebody else might be able to).
 
 To determine the mutability of a URI, parse the second field of the URI (delimited by ':'):
 
  * URI:CHK:...    : immutable
  * URI:LIT:...    : immutable
+ * URI:SSK:...    : mutable, writable
+ * URI:SSK-RO:... : mutable, not writable
  * URI:DIR:...    : mutable, writable
  * URI:DIR-RO:... : mutable, not writable
 
-Note: Currently all files are immutable, and all directories are mutable, but in the future the node type (file or directory) and the mutability will be independent.  It's a bad idea to rely on the node type to determine mutability.
+Note: Currently most files are immutable, and all directories are mutable, but in the future the node type (file or directory) and the mutability will be independent.  It's a bad idea to rely on the node type to determine mutability.
 
 
 ### HTTP methods
@@ -34,13 +36,11 @@ The api relies on four HTTP methods defined as per the standard `GET, POST, PUT,
 Although these are standard HTTP methods, some clients do not offer good support for them.
  * The following clients are known to support all methods:
    * [curl](http://curl.haxx.se/) is an open source client which supports arbitrary methods with the '-X' option.
+   * Python 2.5's stdlib modules (`httplib`, `urllib`) provide good support for arbitrary methods.
  * The following clients do not seem to support either `PUT` and `DELETE`:
    * ...
  * We have not yet determined if the following have support for arbitrary methods:
-   * Common web browsers.
-   * Python 2.5's stdlib:
-     * `httplib`
-     * `urllib`
+   * Common web browsers. In general, HTML forms can only cause GET and POST operations. Javascript might have more abilities.
 
 ### Race Conditions