added a chapter on treeishs

text/17_Git_Treeishes/0_ Git_Treeishes.markdown

@@ -1 +1,113 @@
-##Git Treeishes
+## Git Treeishes ##
+
+There are a number of ways to refer to a particular commit or tree other
+than spelling out the entire 40-character sha.
+
+### Partial Sha ###
+
+If your commit sha is '<code>980e3ccdaac54a0d4de358f3fe5d718027d96aae</code>', git will 
+recognize any of the following identically:
+
+	980e3ccdaac54a0d4de358f3fe5d718027d96aae
+	980e3ccdaac54a0d4
+	980e3cc
+
+As long as the partial sha is unique - it can't be confused with another
+(which is incredibly unlikely if you use at least 5 characters), git will
+expand a partial sha for you.
+
+### Branch, Remote or Tag Name ###
+
+You can always use a branch, remote or tag name instead of a sha, since they
+are simply pointers anyhow.  If your master branch is on the 980e3 commit and
+you've pushed it to origin and have tagged it 'v1.0', then all of the following
+are equivalent:
+
+	980e3ccdaac54a0d4de358f3fe5d718027d96aae
+	origin/master
+	refs/remotes/origin/master
+	master
+	refs/heads/master
+	v1.0
+	refs/tags/v1.0
+
+Which means the following will give you identical output:
+
+	$ git log master
+
+	$ git log refs/tags/v1.0
+
+### Date Spec ###
+
+The Ref Log that git keeps will allow you to do some relative stuff locally, 
+such as: 
+
+	master@{yesterday}
+
+	master@{1 month ago}
+
+Which is shorthand for 'where the master branch head was yesterday', etc. Note
+that this format can result in different shas on different computers, even if
+the master branch is currently pointing to the same place.
+
+### Ordinal Spec ###
+
+This format will give you the Nth previous value of a particular reference.
+For example:
+
+	master@{5}
+
+will give you the 5th prior value of the master head ref.
+
+### Carrot Parent ###
+
+This will give you the Nth parent of a particular commit.  This format is only
+useful on merge commits - commit objects that have more than one direct parent.
+
+	master^2
+
+
+### Tilde Spec ###
+
+The tilde spec will give you the Nth grandparent of a commit object.  For example,
+
+	master~2
+
+will give us the first parent of the first parent of the commit that master 
+points to.  It is equivalent to:
+
+	master^^
+
+You can keep doing this, too.  The following specs will point to the same commit:
+
+	master^^^^^^
+	master~3^~2
+	master~6
+
+### Tree Pointer ###
+
+This disambiguates a commit from the tree that it points to.  If you want the 
+sha that a commit points to, you can add the '^{tree}' spec to the end of it.
+
+	master^{tree}
+
+### Blob Spec ###
+
+If you want the sha of a particular blob, you can add the blob path at the
+end of the treeish, like so:
+
+	master:/path/to/file
+
+### Range ###
+
+Finally, you can specify a range of commits with the range spec.  This will
+give you all the commits between 7b593b5 and 51bea1 (where 51bea1 is most recent),
+excluding 7b593b5 but including 51bea1:
+
+	7b593b5..51bea1
+
+This will include every commit *since* 7b593b:
+
+	7b593b.. 
+
+