diff --git a/TODO.md b/TODO.md
index 9920919..58cf912 100644
--- a/TODO.md
+++ b/TODO.md
@@ -10,7 +10,6 @@
  - Commands to list constants, units, and substitutions
  - Documentation (usage)
  - Documentation (comments)
- - Build.rs for constants
  - Print with highlighting
  - Cleanup tests, test commands, test units, trig/function tests
  - Releases
@@ -64,7 +63,6 @@
  - Print units with powers instead of /
  - HMS for degrees
  - Exact radians, using pi constant?
- - Constant toml file (update doc in units.toml)
  - Weird units: flops, lumen, lux, bel
 
 ## Bonus
diff --git a/buildscript/constants.toml b/buildscript/constants.toml
index 450c1a0..4da68cd 100644
--- a/buildscript/constants.toml
+++ b/buildscript/constants.toml
@@ -1,3 +1,12 @@
+# This file defines all constants daisy knows about.
+# Each is an entry in the `constant` array.
+#
+# Parameters:
+# enum_name: capitalized string. Name of this constant's enum.
+# strings: string or string array. What strings will be parsed as this constant.
+#	The first entry in this array tells daisy how to print this constant.
+# value: value of this constant. Will be evaluated just like user input.
+# unit: true if this constant represents a "fake unit"
 
 [[constant]]
 enum_name = "Pi"
diff --git a/buildscript/units.toml b/buildscript/units.toml
index 12eb3df..db77951 100644
--- a/buildscript/units.toml
+++ b/buildscript/units.toml
@@ -1,5 +1,6 @@
 # This file defines all units daisy knows about.
 # Each unit is an entry in the `unit` array.
+# Some "units" are constants (mph, for example). They are defined in the "constants" file.
 #
 #
 # Basic properties: