diff --git a/fontc/src/lib.rs b/fontc/src/lib.rs
index a57c60025..24cac5db6 100644
--- a/fontc/src/lib.rs
+++ b/fontc/src/lib.rs
@@ -326,7 +326,7 @@ mod tests {
     use fontbe::orchestration::{
         AnyWorkId, Context as BeContext, Glyph, LocaFormatWrapper, WorkId as BeWorkIdentifier,
     };
-    use fontdrasil::types::GlyphName;
+    use fontdrasil::{paths::glyph_file, types::GlyphName};
     use fontir::{
         ir::{self, KernParticipant},
         orchestration::{Context as FeContext, Persistable, WorkId as FeWorkIdentifier},
@@ -818,12 +818,12 @@ mod tests {
     }
 
     fn read_ir_glyph(build_dir: &Path, name: &str) -> ir::Glyph {
-        let raw_glyph = read_file(&build_dir.join(format!("glyph_ir/{name}.yml")));
+        let raw_glyph = read_file(&build_dir.join("glyph_ir").join(glyph_file(name, ".yml")));
         ir::Glyph::read(&mut raw_glyph.as_slice())
     }
 
     fn read_be_glyph(build_dir: &Path, name: &str) -> RawGlyph {
-        let raw_glyph = read_file(&build_dir.join(format!("glyphs/{name}.glyf")));
+        let raw_glyph = read_file(&build_dir.join("glyphs").join(glyph_file(name, ".glyf")));
         let read: &mut dyn Read = &mut raw_glyph.as_slice();
         Glyph::read(read).data
     }
diff --git a/fontdrasil/src/paths.rs b/fontdrasil/src/paths.rs
index 3ea2edc88..83a3a833e 100644
--- a/fontdrasil/src/paths.rs
+++ b/fontdrasil/src/paths.rs
@@ -1,5 +1,45 @@
 pub fn glyph_file(glyph_name: &str, suffix: &str) -> String {
     // TODO handle names that are invalid for the filesystem
     // Ref https://github.com/unified-font-object/ufo-spec/issues/164
-    glyph_name.to_owned() + suffix
+    let mut filename = Vec::new();
+    for ch in glyph_name.chars() {
+        filename.push(ch);
+        if ch == '_' || ch.is_uppercase() {
+            filename.push('_');
+        }
+    }
+    filename.extend(suffix.chars());
+    filename.into_iter().collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::HashSet;
+
+    use super::glyph_file;
+
+    /// <https://github.com/googlefonts/fontc/issues/41>
+    fn assert_unique_for_caseinsensitive_fs(names: &[&str]) {
+        let filenames: HashSet<_> = names
+            .iter()
+            .map(|n| glyph_file(n, ""))
+            .map(|n| n.to_lowercase())
+            .collect();
+        assert_eq!(
+            names.len(),
+            filenames.len(),
+            "{names:?} became {filenames:?}"
+        );
+    }
+
+    #[test]
+    fn lower_and_upper_a() {
+        assert_unique_for_caseinsensitive_fs(&["a", "A"]);
+    }
+
+    #[test]
+    fn adding_underscore_avoids_collisions() {
+        // if we don't add _ to _ the resulting names are identical
+        assert_unique_for_caseinsensitive_fs(&["Aa", "a_a"]);
+    }
 }