diff --git a/gajumaru-core/test/README.md b/gajumaru-core/test/README.md
index 200accb..6bfd9e1 100644
--- a/gajumaru-core/test/README.md
+++ b/gajumaru-core/test/README.md
@@ -13,3 +13,12 @@ In the current directory simply run `zx runlocal` and the report map will be pri
 To run a specific test suite run `zx runlocal [module names]`.
 
 To list module names, run `zx runlocal list`.
+
+
+## Where things are
+
+The test cases are all generated in gmt.erl, generally based on randomized but valid data that are fed to reference Erlang implementations.
+The input to and output from each test is recorded to disk in the `temp/` dir.
+The inputs are then read in by equivalent Java functions and the output is then compared.
+
+For a test case to pass the outputs must be identical.