diff --git a/instrumentation-cheat-sheet.tex b/instrumentation-cheat-sheet.tex
index 082e677..3191fac 100644
--- a/instrumentation-cheat-sheet.tex
+++ b/instrumentation-cheat-sheet.tex
@@ -99,8 +99,9 @@
 \tl_const:Nn  \l_ambitus_bar_height_tl { 0.155 }
 \tl_const:Nn  \l_ambitus_bar_separation_tl { 0.45 } % This is a fraction of the ambitus bar height.
 \tl_new:N     \g_ambitus_y_tl
-% TikZ uses 0.4pt for the default width of a line. Subtract this from \textwidth
-% so the cheat sheet doesn’t extend into the right margin.
+% From https://tikz.dev/tikz-actions#sec-15.3.1, TikZ uses 0.4pt for the default
+% width of a line. Subtract this from \textwidth so the cheat sheet doesn’t
+% extend into the right margin.
 \dim_const:Nn \l_black_key_width_dim { (\textwidth - 0.4pt) / \l_total_note_count_tl }
 
 \tikzset{midi~ note~ number/.style={anchor=mid~ west, minimum~ height=\l_ambitus_bar_height_tl cm, minimum~ width=\l_black_key_width_dim}}