diff --git a/_Sidebar.md b/_Sidebar.md
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@@ -29,6 +29,7 @@
   * [[Sounder]]
     * [[Music Macro language]]
   * [[BOOTSEL mode of RP2040]]
+  * [[require]]
 * Examples
   * [[Microcontroller boards and Keyboards]]
 * Development
diff --git a/require.md b/require.md
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+## Valid version
+
+|Feature|Version|
+|----|----|
+|Loading prebuilt libraries|0.9.14+ (maybe)|
+|Loading external libraries|0.9.23+|
+
+This page explains the loading external libraries feature introduced in 0.9.23.
+
+## Description
+
+CRuby loads external libraries using `Kernel.#require` method.
+Likewise, PicoRuby has `require` method.
+You can reuse a Ruby script by taking advantage of that.
+
+## Gist
+
+- Put your library files in the `/lib` directory in "PRK DRIVE"
+  - It may be easier to understand if you say `$LOAD_PATH` is `["/lib"]`
+- The library file has to have `.rb` or `.mrb` extention. eg: `my_library.rb`
+- `require` method ignores the extension, so just write `require "my_library"`
+- `.rb` file will be compiled into mruby VM code in microcontrller on the fly
+  - A large Ruby file is more likely to cause Out of Memory
+  - In such case, precompled mrb file will help
+  - You can get `my_library.mrb` by `mrbc my_library.rb` or `picorbc my_library.rb`
+    - Make sure to use the same version of the compiler as the PicoRuby in PRK
+    - As of PRK 0.9.23, the corresponding version of mruby (picoruby) is 3.2.x
+
+## Example
+
+```ruby
+# Library for ADNS-5050 optical sensor
+# filepath: /lib/adns5050.rb
+require "mouse"
+require "spi"
+
+class ADNS5050
+  def initialize(sck_pin:, copi_pin:, cipo_pin:, cs_pin:)
+    spi = SPI.new(unit: :BITBANG, sck_pin: sck_pin, copi_pin: copi_pin, cipo_pin: cipo_pin, cs_pin: cs_pin)
+    mouse = Mouse.new(driver: spi)
+    mouse.task do |mouse, keyboard|
+      next if mouse.driver.power_down
+      y, x = mouse.driver.select do |spi|
+        spi.write(0x63) # Motion_Burst
+        spi.read(2).bytes
+      end
+      if 0 < x || 0 < y
+        x = 0x7F < x ? (~x & 0xff) + 1 : -x
+        y = 0x7F < y ? (~y & 0xff) + 1 : -y
+        if keyboard.layer == :lower
+          x = 0 < x ? 1 : (x < 0 ? -1 : x)
+          y = 0 < y ? 1 : (y < 0 ? -1 : y)
+          USB.merge_mouse_report(0, 0, 0, y, -x)
+        else
+          mod = keyboard.modifier
+          if 0 < mod & 0b00100010
+            # Shift key pressed -> Horizontal or Vertical only
+            x.abs < y.abs ? x = 0 : y = 0
+          end
+          if 0 < mod & 0b01000100
+            # Alt key pressed -> Fix the move amount
+            x = 0 < x ? 2 : (x < 0 ? -2 : x)
+            y = 0 < y ? 2 : (y < 0 ? -2 : y)
+          end
+          USB.merge_mouse_report(0, y, x, 0, 0)
+        end
+      end
+    end
+    return mouse
+  end
+end
+```
+
+```ruby
+# filepath: /keyboard.rb
+require "adns5050"
+
+kbd = Keyboard.new
+
+kbd.append ADNS5050.new(sck_pin: 23, copi_pin: 8, cipo_pin: 8, cs_pin: 9)
+
+kbd.start!
+```
+
+## Pro tip
+
+When you make font data for OLED, in general, remember that String objects are more memory-effective than Array objects.
+
+```ruby
+# Array object
+FONT_DATA = [
+  0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+  0x3E, 0x5B, 0x4F, 0x5B, 0x3E, 0x00,
+  ...
+]
+```
+
+```ruby
+# String object
+FONT_DATA = "\x00\x00\x00\x00\x00\x00\x3E\x5B\x4F\x5B\x3E\x00..."
+```
+
+If OOM still occurs, splitting the data may work:
+
+```ruby
+# Split String objects
+FONT_DATA_1 = "\x00\x00\x00\x00\x00\x00\x3E\x5B\x4F\x5B\x3E\x00..."
+FONT_DATA_2 = "\xC0\x00\xDC\xD7\xDE\xDE\xDE\xD7\xDC\x00\xC0\x00..."
+FONT_DATA = FONT_DATA_1 + FONT_DATA_2
+```
+
+Needless to say, precompiled `.mrb` is even more effective.
+