Update basis_spec.txt
diff --git a/spec/basis_spec.txt b/spec/basis_spec.txt
index dd1068c..6f71594 100644
--- a/spec/basis_spec.txt
+++ b/spec/basis_spec.txt
@@ -835,11 +835,13 @@
11.0 Alpha Channels in ETC1S Format Files
-----------------------------------------
-If the ETC1S .basis file has an alpha channel, basis_file_header::m_flags will be logically OR's with
+ETC1S .basis files can have optional alpha channels, stored in odd slices. If any slice needs an alpha channel,
+all slices must have alpha channels. basis_file_header::m_flags will be logically OR's with
cBASISHeaderFlagHasAlphaSlices. Alpha channel ETC1S files will contain two slices for each mipmap level
(or face, or video frame, etc.). The even slices will contain the RGB data, and the odd slices will contain
-the alpha data. A decoder can first decode the RGB data slice, then the next alpha channel slice, or it can
-decode them in parallel using multithreading.
+the alpha data, both stored in ETC1S format. A decoder can first decode the RGB data slice, then the next alpha
+channel slice, or it can decode them in parallel using multithreading. The ETC1S green channel contains the
+alpha values.
12.0 Texture Video
------------------
