diff --git a/doc/manual.org b/doc/manual.org
index 6aa8612c..b48b5172 100644
--- a/doc/manual.org
+++ b/doc/manual.org
@@ -62,7 +62,8 @@ format and the third one describes how to use the =jbuilder= command.
 - *root*: the root is the directory from where Jbuilder can build
   things. Jbuilder knows how to build target that are descendant of
   the root. Anything outside of the tree starting from the root is
-  considered part of the *installed world*
+  considered part of the *installed world*. How the root is determined
+  is explained in [[Finding the root][this section]].
 
 - *workspace*: a workspace is the sub-tree starting from the root. It
   can contain any number of projects that will be built simultaneously
@@ -875,9 +876,44 @@ the root of your project. What you should write instead is:
   (action  (chdir ${ROOT} (ocamllex -o ${@} ${<})))))
 #+end_src
 
+** jbuild-ignore
+
+By default Jbuilder traverses the whole source tree. To ignore a
+sub-tree, simply write a =jbuild-ignore= file in the parent directory
+containing the name of the sub-directories to ignore.
+
+So for instance, if you write =foo= in =src/jbuild-ignore=, then
+=src/foo= won't be traversed and any =jbuild= file it contains will be
+ignored.
+
+=jbuild-ignore= files contain a list of directory names, one per
+line.
+
 * Usage
 
-TODO
+This section describe usage of Jbuilder from the shell.
+
+** Finding the root
+
+The root of the current workspace is determined by looking up specific
+files/directories in the current directory and parent directories.
+
+More precisely, consider the current directory and all its
+ancestors. For instance if you are in =/home/me/code/myproject/src=,
+then this is this set:
+
+- =/home/me/code/myproject/src=
+- =/home/me/code/myproject=
+- =/home/me/code=
+- =/home/me=
+- =/home=
+- =/=
+
+Jbuilder looks for the following entries in all these directories:
+
+1. jbuild-workspace
+2. any file ending with =.install=
+3. =.git= or =.hg=
 
 * Advanced topics
 
diff --git a/src/build_system.ml b/src/build_system.ml
index 91e7dd24..9d7c6f44 100644
--- a/src/build_system.ml
+++ b/src/build_system.ml
@@ -22,7 +22,9 @@ module Rule = struct
   type t =
     { deps         : Pset.t
     ; targets      : Pset.t
-    ; lib_deps     : Build.lib_deps Pmap.t
+    ; (* Keep the arrow around so that we can do more query, such as for finding external
+         library dependencies *)
+      build        : (unit, unit) Build.t
     ; mutable exec : Exec_status.t
     }
 end
@@ -213,7 +215,6 @@ let compile_rule t ~all_targets_by_dir ?(allow_override=false) pre_rule =
   let { Pre_rule. build; targets = target_specs } = pre_rule in
   let deps = Build_interpret.deps build ~all_targets_by_dir in
   let targets = Target.paths target_specs in
-  let lib_deps = Build_interpret.lib_deps build in
 
   if !Clflags.debug_rules then begin
     let f set =
@@ -221,6 +222,7 @@ let compile_rule t ~all_targets_by_dir ?(allow_override=false) pre_rule =
       |> List.map ~f:Path.to_string
       |> String.concat ~sep:", "
     in
+    let lib_deps = Build_interpret.lib_deps build in
     if Pmap.is_empty lib_deps then
       Printf.eprintf "{%s} -> {%s}\n" (f deps) (f targets)
     else
@@ -251,7 +253,7 @@ let compile_rule t ~all_targets_by_dir ?(allow_override=false) pre_rule =
     { Rule.
       deps    = deps
     ; targets = targets
-    ; lib_deps
+    ; build
     ; exec
     }
   in
@@ -384,7 +386,8 @@ let rules_for_targets t targets =
 let all_lib_deps t targets =
   List.fold_left (rules_for_targets t targets) ~init:Pmap.empty
     ~f:(fun acc (_, rule) ->
-      Pmap.merge acc rule.Rule.lib_deps ~f:(fun _ a b ->
+      let lib_deps = Build_interpret.lib_deps rule.Rule.build in
+      Pmap.merge acc lib_deps ~f:(fun _ a b ->
         match a, b with
         | None, None -> None
         | Some a, None -> Some a
@@ -393,7 +396,8 @@ let all_lib_deps t targets =
 
 let all_lib_deps_by_context t targets =
   List.fold_left (rules_for_targets t targets) ~init:[] ~f:(fun acc (_, rule) ->
-    Path.Map.fold rule.Rule.lib_deps ~init:acc ~f:(fun ~key:path ~data:lib_deps acc ->
+    let lib_deps = Build_interpret.lib_deps rule.Rule.build in
+    Path.Map.fold lib_deps ~init:acc ~f:(fun ~key:path ~data:lib_deps acc ->
       match Path.extract_build_context path with
       | None -> acc
       | Some (context, _) -> (context, lib_deps) :: acc))