diff --git a/aom/aom_codec.h b/aom/aom_codec.h
index ce034b3..2a5ef7e 100644
--- a/aom/aom_codec.h
+++ b/aom/aom_codec.h
@@ -183,7 +183,15 @@
 /*!\brief Codec interface structure.
  *
  * Contains function pointers and other data private to the codec
- * implementation. This structure is opaque to the application.
+ * implementation. This structure is opaque to the application. Common
+ * functions used with this structure:
+ *   - aom_codec_iface_name: get the name of the codec
+ *   - aom_codec_get_caps: returns the capabilities of the codec (see
+ *     aom_encoder.h for more details)
+ *   - aom_codec_enc_config_default: generate the default config to use
+ *     when initializing the encoder
+ *   - aom_codec_dec_init, aom_codec_enc_init: initialize the codec context
+ *     structure (see documentation on aom_codec_ctx for more information).
  */
 typedef const struct aom_codec_iface aom_codec_iface_t;
 
diff --git a/common/tools_common.h b/common/tools_common.h
index d9a68f0..1ed0045 100644
--- a/common/tools_common.h
+++ b/common/tools_common.h
@@ -144,20 +144,103 @@
 
 int read_yuv_frame(struct AvxInputContext *input_ctx, aom_image_t *yuv_frame);
 
+///////////////////////////////////////////////////////////////////////////////
+// A description of the interfaces used to access the AOM codecs
+///////////////////////////////////////////////////////////////////////////////
+//
+// There are three levels of interfaces used to access the AOM codec: the
+// AVXInterface, the aom_codec_iface, and the aom_codec_ctx. Each of these
+// is described in detail here.
+//
+//
+// 1. AVXInterface
+//    (Related files: common/tools_common.c,  common/tools_common.h)
+//
+// The high-level interface to the AVx encoders / decoders. Each AvxInterface
+// contains the name of the codec (e.g., "av1"), the four character code
+// associated with it, and a function pointer to the actual interface (see the
+// documentation on aom_codec_iface_t for more info). This API
+// is meant for lookup / iteration over all known codecs.
+//
+// For the encoder, call get_aom_encoder_by_name(...) if you know the name
+// (e.g., "av1"); to iterate over all known encoders, use
+// get_aom_encoder_count() and get_aom_encoder_by_index(i). To get the
+// encoder specifically for large scale tile encoding, use
+// get_aom_lst_encoder().
+//
+// For the decoder, similar functions are available. There is also a
+// get_aom_decoder_by_fourcc(fourcc) to get the decoder based on the four
+// character codes.
+//
+// The main purpose of the AVXInterface is to get a reference to the
+// aom_codec_interface_t, pointed to by its codec_interface variable.
+//
+//
+// 2. aom_codec_iface_t
+//    (Related files: aom/aom_codec.h, aom/src/aom_codec.c,
+//    aom/internal/aom_codec_internal.h, av1/av1_cx_iface.c,
+//    av1/av1_dx_iface.c)
+//
+// Used to initialize the codec context, which contains the configuration for
+// for modifying the encoder/decoder during run-time. See the documentation of
+// aom/aom_codec.h for more details. For the most part, users will call the
+// helper functions listed there, such as aom_codec_iface_name,
+// aom_codec_get_caps, etc., to interact with it.
+//
+// The main purpose of the aom_codec_iface_t is to provide a way to generate
+// a default codec config, find out what capabilities the implementation has,
+// and create an aom_codec_ctx_t (which is actually used to interact with the
+// codec).
+//
+// Note that the implementations of the aom_codec_iface_t are located in
+// av1/av1_cx_iface.c and av1/av1_dx_iface.c
+//
+//
+// 3. aom_codec_ctx_t
+//  (Related files: aom/aom_codec.h, av1/av1_cx_iface.c, av1/av1_dx_iface.c,
+//   aom/aomcx.h, aom/aomdx.h, aom/src/aom_encoder.c, aom/src/aom_decoder.c)
+//
+// The actual interface between user code and the codec. It stores the name
+// of the codec, a pointer back to the aom_codec_iface_t that initialized it,
+// initialization flags, a config for either encoder or the decoder, and a
+// pointer to internal data.
+//
+// The codec is configured / queried through calls to aom_codec_control,
+// which takes a control code (listed in aomcx.h and aomdx.h) and a parameter.
+// In the case of "getter" control codes, the parameter is modified to have
+// the requested value; in the case of "setter" control codes, the codec's
+// configuration is changed based on the parameter. Note that a aom_codec_err_t
+// is returned, which indicates if the operation was successful or not.
+//
+// Note that for the encoder, the aom_codec_alg_priv_t points to the
+// the aom_codec_alg_priv structure in av1/av1_cx_iface.c, and for the decoder,
+// the struct in av1/av1_dx_iface.c. Variables such as AV1_COMP cpi are stored
+// here and also used in the core algorithm.
+//
+// At the end, aom_codec_destroy should be called for each initialized
+// aom_codec_ctx_t.
+
 typedef struct AvxInterface {
   const char *const name;
   const uint32_t fourcc;
+  // Pointer to a function of zero arguments that returns an aom_codec_iface_t
+  // pointer. E.g.:
+  //   aom_codec_iface_t *codec = interface->codec_interface();
   aom_codec_iface_t *(*const codec_interface)();
 } AvxInterface;
 
 int get_aom_encoder_count(void);
+// Lookup the interface by index -- it must be the case that
+// i < get_aom_encoder_count()
 const AvxInterface *get_aom_encoder_by_index(int i);
+// Lookup the interface by name -- returns NULL if no match.
 const AvxInterface *get_aom_encoder_by_name(const char *name);
 const AvxInterface *get_aom_lst_encoder(void);
 
 int get_aom_decoder_count(void);
 const AvxInterface *get_aom_decoder_by_index(int i);
 const AvxInterface *get_aom_decoder_by_name(const char *name);
+// Lookup the interface by the fourcc -- returns NULL if no match.
 const AvxInterface *get_aom_decoder_by_fourcc(uint32_t fourcc);
 
 void aom_img_write(const aom_image_t *img, FILE *file);