diff --git a/src/keyboard.h b/src/keyboard.h
index 0379051..3e6158a 100644
--- a/src/keyboard.h
+++ b/src/keyboard.h
@@ -9,5 +9,5 @@
  * ------------------------------------------------------------------------- */
 
 
-#include KEYBOARD
+#include MAKEFILE_KEYBOARD
 
diff --git a/src/keyboard/ergodox/layout.h b/src/keyboard/ergodox/layout.h
index 84dca27..dcc8825 100644
--- a/src/keyboard/ergodox/layout.h
+++ b/src/keyboard/ergodox/layout.h
@@ -16,7 +16,7 @@
 
 	#include "matrix.h"  // for number of rows and columns
 
-	#include KEYBOARD_LAYOUT  // for number of layers
+	#include MAKEFILE_KEYBOARD_LAYOUT  // for number of layers
 
 	extern uint8_t
 			kb_layout        [KB_LAYERS][KB_ROWS][KB_COLUMNS];
diff --git a/src/keyboard/ergodox/mcp23018.c b/src/keyboard/ergodox/mcp23018.c
index 9af7fd7..2be6cdd 100644
--- a/src/keyboard/ergodox/mcp23018.c
+++ b/src/keyboard/ergodox/mcp23018.c
@@ -10,7 +10,7 @@
 #include <util/twi.h>
 
 #include "lib/_data-types.h"
-#include "lib/_teensy-2-0/twi.h"  // `TWI_FREQ` defined in "teensy-2-0.c"
+#include "lib/_twi.h"  // `TWI_FREQ` defined in "teensy-2-0.c"
 
 #define KEYBOARD_INCLUDE_PRIVATE
 #include "matrix.h"
diff --git a/src/keyboard/ergodox/teensy-2-0.c b/src/keyboard/ergodox/teensy-2-0.c
index 0a7a6c5..89eda5b 100644
--- a/src/keyboard/ergodox/teensy-2-0.c
+++ b/src/keyboard/ergodox/teensy-2-0.c
@@ -11,7 +11,7 @@
 
 #include "lib/_data-types.h"
 #define TWI_FREQ 400000
-#include "lib/_teensy-2-0/twi.h"
+#include "lib/_twi.h"
 
 #define KEYBOARD_INCLUDE_PRIVATE
 #include "matrix.h"
diff --git a/src/lib/_twi.h b/src/lib/_twi.h
new file mode 100644
index 0000000..6803dfa
--- /dev/null
+++ b/src/lib/_twi.h
@@ -0,0 +1,20 @@
+/* ----------------------------------------------------------------------------
+ * TWI (I2C) : exports
+ *
+ * Code specific to different development boards is used by modifying a
+ * variable in the makefile.
+ * ----------------------------------------------------------------------------
+ * Copyright (c) 2012 Ben Blazak <benblazak.dev@gmail.com>
+ * Released under The MIT License (MIT) (see "license.md")
+ * Project located at <https://github.com/benblazak/ergodox-firmware>
+ * ------------------------------------------------------------------------- */
+
+
+#define str(s) #s                              // stringify
+#define expstr(s) str(s)                       // expand -> stringify
+#define inc expstr(_twi/MAKEFILE_BOARD.h)  // inc(lude)
+#include inc
+#undef str
+#undef expstr
+#undef inc
+
diff --git a/src/lib/_teensy-2-0/twi.c b/src/lib/_twi/teensy-2-0.c
similarity index 93%
rename from src/lib/_teensy-2-0/twi.c
rename to src/lib/_twi/teensy-2-0.c
index 2278aff..41b1949 100644
--- a/src/lib/_teensy-2-0/twi.c
+++ b/src/lib/_twi/teensy-2-0.c
@@ -3,7 +3,8 @@
  *
  * - This is mostly straight from the datasheet, section 20.6.6, figure 20-11
  *   (the code example in C), and section 20.8.1, figure 20-12
- * - Also see the documentation for `<util/twi.h>` at <http://www.nongnu.org/avr-libc/user-manual/group__util__twi.html#ga8d3aca0acc182f459a51797321728168>
+ * - Also see the documentation for `<util/twi.h>` at
+ *   <http://www.nongnu.org/avr-libc/user-manual/group__util__twi.html#ga8d3aca0acc182f459a51797321728168>
  *
  * Some other (more complete) TWI libraries for the Teensy 2.0 (and other Atmel
  * processors):
@@ -23,7 +24,7 @@
 #include <util/delay.h> //dbg
 #include <util/twi.h>
 
-#include "twi.h"
+#include "../_twi.h"
 
 
 void twi_init(void) {
diff --git a/src/lib/_teensy-2-0/twi.h b/src/lib/_twi/teensy-2-0.h
similarity index 100%
rename from src/lib/_teensy-2-0/twi.h
rename to src/lib/_twi/teensy-2-0.h
diff --git a/src/lib/_teensy-2-0/twi.md b/src/lib/_twi/teensy-2-0.md
similarity index 96%
rename from src/lib/_teensy-2-0/twi.md
rename to src/lib/_twi/teensy-2-0.md
index eaa880e..ec511cb 100644
--- a/src/lib/_teensy-2-0/twi.md
+++ b/src/lib/_twi/teensy-2-0.md
@@ -1,4 +1,4 @@
-# Documentation : Teensy 2.0 I&sup2;C
+# Documentation : I&sup2;C : Teensy 2.0
 
 ## I&sup2;C Status Codes (for Master modes)
 
diff --git a/src/makefile b/src/makefile
index ce5673d..268fc11 100644
--- a/src/makefile
+++ b/src/makefile
@@ -15,8 +15,9 @@
 
 TARGET   = firmware
 FORMAT   = ihex
-KEYBOARD = ergodox   # see "src/keyboard" for what's available
-LAYOUT   = qwerty    # see "src/keyboard/*/layout" for what's available
+BOARD    = teensy-2-0  # see the libraries you're using for what's available
+KEYBOARD = ergodox     # see "src/keyboard" for what's available
+LAYOUT   = qwerty      # see "src/keyboard/*/layout" for what's available
 
 # firmware stuff
 SRC  = $(wildcard *.c)
@@ -46,8 +47,9 @@ CFLAGS += -DF_CPU=16000000  # processor frequency; must match initialization
 			    #   in source
 CFLAGS += -I.               # search for includes in the current directory
 # . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-CFLAGS += -DKEYBOARD='"keyboard/$(strip $(KEYBOARD)).h"'
-CFLAGS += -DKEYBOARD_LAYOUT='"layout/$(strip $(LAYOUT)).h"'
+CFLAGS += -DMAKEFILE_BOARD='$(strip $(BOARD))'
+CFLAGS += -DMAKEFILE_KEYBOARD='"keyboard/$(strip $(KEYBOARD)).h"'
+CFLAGS += -DMAKEFILE_KEYBOARD_LAYOUT='"layout/$(strip $(LAYOUT)).h"'
 # . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
 CFLAGS += -std=gnu99  # use C99 plus GCC extensions
 CFLAGS += -Os         # optimize for size