diff --git a/Changes.md b/Changes.md
index b92342e1dc..21a0abed6e 100644
--- a/Changes.md
+++ b/Changes.md
@@ -1,11 +1,57 @@
 # Build System Changes for Android.mk Writers
 
+## Removing '/' from Valid Module Names {#name_slash}
+
+The build system uses module names in path names in many places. Having an
+extra '/' or '../' being inserted can cause problems -- and not just build
+breaks, but stranger invalid behavior.
+
+In every case we've seen, the fix is relatively simple: move the directory into
+`LOCAL_MODULE_RELATIVE_PATH` (or `LOCAL_MODULE_PATH` if you're still using it).
+If this causes multiple modules to be named the same, use unique module names
+and `LOCAL_MODULE_STEM` to change the installed file name:
+
+``` make
+include $(CLEAR_VARS)
+LOCAL_MODULE := ver1/code.bin
+LOCAL_MODULE_PATH := $(TARGET_OUT_ETC)/firmware
+...
+include $(BUILD_PREBUILT)
+
+include $(CLEAR_VARS)
+LOCAL_MODULE := ver2/code.bin
+LOCAL_MODULE_PATH := $(TARGET_OUT_ETC)/firmware
+...
+include $(BUILD_PREBUILT)
+```
+
+Can be rewritten as:
+
+```
+include $(CLEAR_VARS)
+LOCAL_MODULE := ver1_code.bin
+LOCAL_MODULE_STEM := code.bin
+LOCAL_MODULE_PATH := $(TARGET_OUT_VENDOR)/firmware/ver1
+...
+include $(BUILD_PREBUILT)
+
+include $(CLEAR_VARS)
+LOCAL_MODULE := ver2_code.bin
+LOCAL_MODULE_STEM := code.bin
+LOCAL_MODULE_PATH := $(TARGET_OUT_VENDOR)/firmware/ver2
+...
+include $(BUILD_PREBUILT)
+```
+
+You just need to make sure that any other references (`PRODUCT_PACKAGES`,
+`LOCAL_REQUIRED_MODULES`, etc) are converted to the new names.
+
 ## Valid Module Names {#name}
 
 We've adopted lexical requirements very similar to [Bazel's
 requirements](https://docs.bazel.build/versions/master/build-ref.html#name) for
 target names. Valid characters are `a-z`, `A-Z`, `0-9`, and the special
-characters `_.+-=,@~/`. This currently applies to `LOCAL_PACKAGE_NAME`,
+characters `_.+-=,@~`. This currently applies to `LOCAL_PACKAGE_NAME`,
 `LOCAL_MODULE`, and `LOCAL_MODULE_SUFFIX`, and `LOCAL_MODULE_STEM*`.
 
 Many other characters already caused problems if you used them, so we don't
diff --git a/core/definitions.mk b/core/definitions.mk
index 07576f9529..9c7f8b6489 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -3488,24 +3488,26 @@ upper :=
 ###########################################################
 ## Verify module name meets character requirements:
 ##   a-z A-Z 0-9
-##   _.+-=,@~/
+##   _.+-=,@~
 ##
-## This is equivalent to bazel's target name restrictions:
+## This is a subset of bazel's target name restrictions:
 ##   https://docs.bazel.build/versions/master/build-ref.html#name
 ###########################################################
 define verify-module-name
+$(if $(filter-out $(LOCAL_MODULE),$(subst /,,$(LOCAL_MODULE))), \
+  $(call pretty-warning,Module name contains a /$(comma) use LOCAL_MODULE_STEM and LOCAL_MODULE_RELATIVE_PATH instead)) \
 $(if $(call _invalid-name-chars,$(LOCAL_MODULE)), \
   $(call pretty-error,Invalid characters in module name: $(call _invalid-name-chars,$(LOCAL_MODULE))))
 endef
 define _invalid-name-chars
-$(subst /,,$(subst _,,$(subst .,,$(subst +,,$(subst -,,$(subst =,,$(subst $(comma),,$(subst @,,$(subst ~,,$(subst 0,,$(subst 1,,$(subst 2,,$(subst 3,,$(subst 4,,$(subst 5,,$(subst 6,,$(subst 7,,$(subst 8,,$(subst 9,,$(subst a,,$(subst b,,$(subst c,,$(subst d,,$(subst e,,$(subst f,,$(subst g,,$(subst h,,$(subst i,,$(subst j,,$(subst k,,$(subst l,,$(subst m,,$(subst n,,$(subst o,,$(subst p,,$(subst q,,$(subst r,,$(subst s,,$(subst t,,$(subst u,,$(subst v,,$(subst w,,$(subst x,,$(subst y,,$(subst z,,$(call to-lower,$(1)))))))))))))))))))))))))))))))))))))))))))))))
+$(subst _,,$(subst .,,$(subst +,,$(subst -,,$(subst =,,$(subst $(comma),,$(subst @,,$(subst ~,,$(subst 0,,$(subst 1,,$(subst 2,,$(subst 3,,$(subst 4,,$(subst 5,,$(subst 6,,$(subst 7,,$(subst 8,,$(subst 9,,$(subst a,,$(subst b,,$(subst c,,$(subst d,,$(subst e,,$(subst f,,$(subst g,,$(subst h,,$(subst i,,$(subst j,,$(subst k,,$(subst l,,$(subst m,,$(subst n,,$(subst o,,$(subst p,,$(subst q,,$(subst r,,$(subst s,,$(subst t,,$(subst u,,$(subst v,,$(subst w,,$(subst x,,$(subst y,,$(subst z,,$(call to-lower,$(1))))))))))))))))))))))))))))))))))))))))))))))
 endef
 .KATI_READONLY := verify-module-name _invalid-name-chars
 
 ###########################################################
 ## Verify module stem meets character requirements:
 ##   a-z A-Z 0-9
-##   _.+-=,@~/
+##   _.+-=,@~
 ##
 ## This is a subset of bazel's target name restrictions:
 ##   https://docs.bazel.build/versions/master/build-ref.html#name
@@ -3513,7 +3515,9 @@ endef
 ## $(1): The module stem variable to check
 ###########################################################
 define verify-module-stem
+$(if $(filter-out $($(1)),$(subst /,,$($(1)))), \
+  $(call pretty-warning,Module stem \($(1)\) contains a /$(comma) use LOCAL_MODULE_RELATIVE_PATH instead)) \
 $(if $(call _invalid-name-chars,$($(1))), \
-  $(call pretty-error,Invalid characters in module stem ($(1)): $(call _invalid-name-chars,$($(1)))))
+  $(call pretty-error,Invalid characters in module stem \($(1)\): $(call _invalid-name-chars,$($(1)))))
 endef
 .KATI_READONLY := verify-module-stem