diff --git a/zk/ESP_ERROR_CHECK.md b/zk/ESP_ERROR_CHECK.md
new file mode 100644
index 0000000..117022b
--- /dev/null
+++ b/zk/ESP_ERROR_CHECK.md
@@ -0,0 +1,13 @@
+---
+tags:
+  - C
+  - ESP32
+  - ESP-IDF
+---
+
+A macro for handling errors arising from the ESP-IDF API's functions.
+
+If a function returns anything other than `ESP_OK`, the macro prints a detailed
+error message and then calls `abort()` to halt execution.
+
+To achieve the same without aborting use `ESP_ERROR_CHECK_WITHOUT_ABORT`.
diff --git a/zk/Macros_in_C.md b/zk/Macros_in_C.md
new file mode 100644
index 0000000..2b37f99
--- /dev/null
+++ b/zk/Macros_in_C.md
@@ -0,0 +1,38 @@
+---
+tags:
+  - C
+---
+
+Think of them as a find and replace that takes place before compilation.
+
+They are ephemeral and never make it into the compiled code.
+
+With a macro, the computation happens "inline" without having to call functions
+which make costly alterations to the stack.
+
+A function and function call such as:
+
+```c
+int square(int x) { return x * x; }
+int a = square(5);
+```
+
+Can be reduced to a macro:
+
+```c
+#define SQUARE(x) ((x) * (x))
+int a = SQUARE(5);
+```
+
+Which, because it uses `#` is processed at the
+[pre-processor compilation stage](./C_compilation_process.md), becomes:
+
+```c
+int a = ((5) * (5));
+```
+
+after pre-processing but before compilation.
+
+## Syntax
+
+`#define` always creates a macro but there are different types.
diff --git a/zk/Objects_in_C.md b/zk/Objects_in_C.md
new file mode 100644
index 0000000..454a045
--- /dev/null
+++ b/zk/Objects_in_C.md
@@ -0,0 +1,18 @@
+---
+tags: ["C"]
+---
+
+Basically, a technical term from the C standard for anything stored in memory.
+(In practice, interchangeable with 'variable' or 'memory'.)
+
+Inclusive of variables, structs, [arrays](./Arrays_in_C.md), array entities
+`arr[2]` etc.
+
+To be distinguished from entities which do not exist in memory such as type
+definitions, [macros](./Macros_in_C.md'), things that are handled by the
+pre-processor.
+
+Where "memory" can be thought of **anything that survives compilation**.
+
+They have nothing to do with "complex data types" that we would associate with
+OOP.
diff --git a/zk/Programming_ESP32_with_Expressif_IoT_Development_Framework.md b/zk/Programming_ESP32_with_Expressif_IoT_Development_Framework.md
index 8322b65..b76a086 100644
--- a/zk/Programming_ESP32_with_Expressif_IoT_Development_Framework.md
+++ b/zk/Programming_ESP32_with_Expressif_IoT_Development_Framework.md
@@ -3,6 +3,7 @@ tags:
   - micro-controllers
   - C
   - ESP32
+  - ESP-IDF
 ---
 
 > The ESP-IDF is the SDK, provided by Expressif, for programming the ESP32 with