diff --git a/README.md b/README.md
index a79e868..829980d 100644
--- a/README.md
+++ b/README.md
@@ -2,33 +2,35 @@
 A place for assorted code ideas for MicroPython. Most are targeted at the
 Pyboard variants.
 
+# 0. Index
+
  1. [Installation guides](./README.md#1-installation-guides)  
   1.1 [Installing MicroPython libraries](./README.md#11-installing-micropython-libraries)  
   1.2 [Fastbuild](./README.md#12-fastbuild) Build scripts and udev rules  
-  1.3 [Installing PicoWeb](./README.md#13-installing-picoweb)  
+  1.3 [Installing PicoWeb](./README.md#13-installing-picoweb) For users of official firmware  
   1.4 [Buildcheck](./README.md#14-buildcheck) Check firmware build date  
  2. [Hardware information and drivers](./README.md#2-hardware-information-and-drivers)  
-  2.1 [ESP32](./README.md#21-esp32)  
+  2.1 [ESP32](./README.md#21-esp32) Pinout and notes on the reference board  
   2.2 [SSD1306](./README.md#22-ssd1306) Write large fonts to the SSD1306  
   2.3 [Pyboard D](./README.md#23-pyboard-d) Assorted scraps of information  
   2.4 [DS3231 precision RTC](./README.md#24-ds3231-precision-rtc) Use cheap hardware to calibrate Pyboard RTC  
  3. [Essays](./README.md#3-essays) General thoughts  
   3.1 [Resilient](./README.md#31-resilient) A guide to writing resilient WiFi code  
-  3.2 [Serialisation](./README.md#32-serialisation) MicroPython's four serialisation libraries  
-  3.3 [Measurement of relative timing and phase of fast analog signals](./README.md#33-measurement-of-relative-timing-and-phase-of-fast-analog-signals)  
+  3.2 [Serialisation](./README.md#32-serialisation) Review of MicroPython's four serialisation libraries  
+  3.3 [Measurement of relative timing and phase of fast analog signals](./README.md#33-measurement-of-relative-timing-and-phase-of-fast-analog-signals) For Pyboard.  
  4. [Code samples](./README.md#4-code-samples) Samples prefixed Pyboard are Pyboard specific  
   4.1 [Pyboard Mutex](./README.md#41-pyboard-mutex) Share data between threads and ISR's.  
   4.2 [Pyboard watchdog](./README.md#42-pyboard-watchdog) Access a Pyboard hardware WDT  
   4.3 [Software Watchdog](./README.md#43-software-watchdog) Cross-platform soft WDT  
   4.4 [Reverse](./README.md#44-reverse) Reversal algorithms for bytearrays  
-  4.5 [Timed function](./README.md#45-timed-function)  
-  4.6 [ESP8266 MQTT benchmark](./README.md#46-esp8266-mqtt-benchmark) Test performance of official library  
+  4.5 [Timed function](./README.md#45-timed-function) Time execution with a decorator  
+  4.6 [ESP8266 MQTT benchmark](./README.md#46-esp8266-mqtt-benchmark) Test performance of MQTT with official library  
   4.7 [Rotary incremental encoder](./README.md#47-rotary-incremental-encoder) Fast, simple, proven algorithm  
   4.8 [A pseudo random number generator](./README.md#48-a-pseudo-random-number-generator)  
-  4.9 [Verifying incrementing sequences](./README.md#49-verifying-incrementing-sequences)  
+  4.9 [Verifying incrementing sequences](./README.md#49-verifying-incrementing-sequences) Test communications drivers  
   4.10 [Bitmaps](./README.md#410-bitmaps) Non-allocating ways to access bitmaps  
-  4.11 [Functors and singletons](./README.md#411-functors-and-singletons)  
-  4.12 [A Pyboard power meter](./README.md#412-a-pyboard-power-meter)  
+  4.11 [Functors and singletons](./README.md#411-functors-and-singletons) Useful decorators  
+  4.12 [A Pyboard power meter](./README.md#412-a-pyboard-power-meter) One of my own projects  
 
 # 1. Installation guides
 
@@ -88,6 +90,8 @@ from the DS3231. Calibration to high precision may be achieved in five minutes.
 
 The drivers are [documented here](./DS3231/README.md).
 
+##### [Index](./README.md#0-index)
+
 # 3. Essays
 
 ## 3.1 Resilient
@@ -106,6 +110,8 @@ tutorial on a Protocol Buffer library.
 This describes ways of using the Pyboard to perform precision measurements of
 analog signals of up to around 50KHz. It is documented [here](./phase/README.md).
 
+##### [Index](./README.md#0-index)
+
 # 4. Code samples
 
 ## 4.1 Pyboard mutex
@@ -137,6 +143,8 @@ bit words.
 Time a function's execution using a [decorator](./timed_function/timed_func.py)
 and implement timeouts using a [closure](./timed_function/timeout.py).
 
+##### [Index](./README.md#0-index)
+
 ## 4.6 ESP8266 MQTT benchmark
 
 [This benchmark](./ESP8266/benchmark.py) tests the performance of MQTT by
@@ -184,7 +192,13 @@ See [random.py](./random/random.py) for usage and timing documentation.
 
 ## 4.9 Verifying incrementing sequences
 
+When testing communications applications it is often necessary to check for
+missing, duplicated, or out-of-order messages. To do this, the transmitter test
+script ensures that messages include an incrementing message number. The
+receiver script verifies the sequence. [The CheckMid class](./sequence/check_mid.py)
+does this, also detecting transmitter reboots
 
+##### [Index](./README.md#0-index)
 
 ## 4.10 Bitmaps
 
@@ -234,6 +248,8 @@ enabling it to provide information on power factor and to work with devices
 which generate as well as consume power. It uses the official LCD160CR display
 as a touch GUI interface. It is documented [here](./power/README.md).
 
+##### [Index](./README.md#0-index)
+
 # License
 
 Any code placed here is released under the MIT License (MIT).