diff --git a/README.md b/README.md
index 08c9fd0..bd7e29f 100644
--- a/README.md
+++ b/README.md
@@ -183,9 +183,12 @@ export GTK_IM_MODULE=strans
 GLFW_IM_MODULE=ibus kitty
 ```
 
-Building and installing `gtk/im-strans.so` requires GTK development files;
-the install target uses GTK's own module directory, which packagers and
-cross builds override with `GTK_MODULE_DIR`.
+Building `gtk/im-strans.so` requires GTK development files; installing one
+built elsewhere, in the container for instance, does not.  The install
+target uses `GTK_MODULE_DIR` when it is set, else GTK's pkg-config
+variables, else the directory the installed `gtk-query-immodules-3.0`
+reports.  Packagers and cross builds should set `GTK_MODULE_DIR`, and a
+prebuilt module must still match the target architecture and library ABI.
 
 `DESTDIR` staging is supported and deliberately skips the host module-cache
 update. A native install needs the GTK runtime query utility so it can refresh
diff --git a/gtk/Makefile b/gtk/Makefile
index f357eec..a9d3277 100644
--- a/gtk/Makefile
+++ b/gtk/Makefile
@@ -7,8 +7,14 @@ GTK_QUERY_IMMODULES ?= gtk-query-immodules-3.0
 GTK_LIBDIR ?= $(shell $(PKG_CONFIG) --variable=libdir gtk+-3.0 2>/dev/null)
 GTK_BINARY_VERSION ?= $(shell $(PKG_CONFIG) \
 	--variable=gtk_binary_version gtk+-3.0 2>/dev/null)
-GTK_MODULE_DIR ?= $(strip $(if $(GTK_BINARY_VERSION),\
-	$(GTK_LIBDIR)/gtk-3.0/$(GTK_BINARY_VERSION)/immodules))
+# Without GTK development files, ask the runtime where its modules live.
+GTK_RUNTIME_MODULE_DIR = $(shell GTK_PATH= GTK_IM_MODULE_FILE= \
+	$(GTK_QUERY_IMMODULES) 2>/dev/null | \
+	awk -F '"' '/^"\// { sub("/[^/]*$$", "", $$2); print $$2; exit }')
+GTK_MODULE_DIR ?= $(strip $(if \
+	$(and $(strip $(GTK_LIBDIR)),$(strip $(GTK_BINARY_VERSION))),\
+	$(GTK_LIBDIR)/gtk-3.0/$(GTK_BINARY_VERSION)/immodules,\
+	$(GTK_RUNTIME_MODULE_DIR)))
 
 all: $(PROG)