diff --git a/legacy/application-abi.md b/legacy/application-abi.md
index a87f9d27..39373da6 100644
--- a/legacy/application-abi.md
+++ b/legacy/application-abi.md
@@ -51,6 +51,13 @@ Regardless of the kind, all modules accessing WASI APIs also export a table
 with the name `__indirect_function_table`. Function pointers in WASI API calls
 are relative to this table's index space.
 
+When `_start` or `_initialize` is called, environments shall provide file
+descriptors with indices 0, 1, and 2 representing stream resources for
+standard input, standard output, and standard error. Environments may provide
+additional "preopen" file descriptors that can be inspected with
+`fd_prestat_get` and `fd_prestat_dir_name`. These resources may be closed at
+any time.
+
 Environments shall not access exports named `__heap_base` or `__data_end`.
 Toolchains are encouraged to avoid providing these exports.