diff --git a/src/BizHawk.Tests/PlatformTests/Common.PathExtensions/PathExtensionTests.cs b/src/BizHawk.Tests/PlatformTests/Common.PathExtensions/PathExtensionTests.cs
new file mode 100644
index 0000000000..297500a9ac
--- /dev/null
+++ b/src/BizHawk.Tests/PlatformTests/Common.PathExtensions/PathExtensionTests.cs
@@ -0,0 +1,104 @@
+using BizHawk.Common;
+using BizHawk.Common.PathExtensions;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+using PE = BizHawk.Common.PathExtensions.PathExtensions;
+
+namespace BizHawk.Tests.Common.PathExtensions
+{
+	[TestClass]
+	public class PathExtensionTests
+	{
+		[TestMethod]
+		public void TestNullability()
+		{
+			var p = OSTailoredCode.IsUnixHost ? "/" : @"C:\";
+
+			Assert.IsFalse(PE.IsSubfolderOf(childPath: null, parentPath: p));
+			Assert.IsFalse(PE.IsSubfolderOf(childPath: p, parentPath: null));
+			Assert.IsFalse(PE.IsSubfolderOf(childPath: null, parentPath: null));
+
+			Assert.IsNull(PE.GetRelativePath(fromPath: null, toPath: p));
+			Assert.IsNull(PE.GetRelativePath(fromPath: p, toPath: null));
+			Assert.IsNull(PE.GetRelativePath(fromPath: null, toPath: null));
+
+			Assert.IsNull(PE.MakeRelativeTo(absolutePath: null, basePath: p));
+			Assert.IsNull(PE.MakeRelativeTo(absolutePath: p, basePath: null));
+			Assert.IsNull(PE.MakeRelativeTo(absolutePath: null, basePath: null));
+		}
+
+		[TestMethod]
+		[DataRow(true, "/usr/share", "/usr")]
+		[DataRow(false, "/usr/share", "/bin")]
+		[DataRow(false, "/usr", "/usr/share")]
+		[DataRow(true, "/usr", "/usr")]
+		[DataRow(true, "/usr", "/usr/")]
+		[DataRow(false, "/etc/rmdir", "/etc/rm")] // not naive StartsWith; these don't exist but the implementation uses `realpath -m` so they will be classed as two real and distinct dirs
+		[DataRow(true, "/usr/lib64", "/usr/lib")] // symlink to same dir
+		[DataRow(true, "/usr/lib64/gconv", "/usr/lib")] // same symlink, checking child
+		public void TestIsSubfolderOfUnix(bool expectedIsSubfolder, string childPath, string parentPath)
+		{
+			PlatformTestUtils.OnlyRunOnRealUnix();
+
+			Assert.AreEqual(expectedIsSubfolder, childPath.IsSubfolderOf(parentPath));
+		}
+
+		[TestMethod]
+		[DataRow(true, @"C:\Users\Public", @"C:\Users")]
+		[DataRow(false, @"C:\Users\Public", @"C:\Program Files")]
+		[DataRow(false, @"C:\Users", @"C:\Users\Public")]
+		[DataRow(true, @"C:\Users", @"C:\Users")]
+		[DataRow(true, @"C:\Users", @"C:\Users\")]
+		[DataRow(false, @"C:\Program Files (x86)", @"C:\Program Files")] // not naive StartsWith
+		public void TestIsSubfolderOfWindows(bool expectedIsSubfolder, string childPath, string parentPath)
+		{
+			PlatformTestUtils.OnlyRunOnWindows();
+
+			Assert.AreEqual(expectedIsSubfolder, childPath.IsSubfolderOf(parentPath));
+		}
+
+#if false // don't bother, GetRelativePath simply calls MakeRelativeTo on Unix
+		public void TestGetRelativePathUnix() {}
+#endif
+
+		[TestMethod]
+		[DataRow("./share", "/usr/share", "/usr")]
+		[DataRow(".", "/usr", "/usr")]
+		[DataRow("/usr", "/usr", "/usr/share")] // not `..`
+		[DataRow("/usr/bin", "/usr/bin", "/usr/share")] // not `../bin`
+		[DataRow("/bin", "/bin", "/usr/share")] // not `../../bin`
+		public void TestMakeRelativeToUnix(string expectedRelPath, string absolutePath, string basePath)
+		{
+			PlatformTestUtils.OnlyRunOnRealUnix();
+
+			Assert.AreEqual(expectedRelPath, absolutePath.MakeRelativeTo(basePath));
+		}
+
+		[TestMethod]
+		[DataRow(@".\SysWOW64", @"C:\Windows\SysWOW64", @"C:\Windows")]
+		[DataRow(@".", @"C:\Windows", @"C:\Windows")]
+		[DataRow(@"..", @"C:\Windows", @"C:\Windows\SysWOW64")]
+		[DataRow(@"..\System32", @"C:\Windows\System32", @"C:\Windows\SysWOW64")]
+		[DataRow(@"..\..\Program Files", @"C:\Program Files", @"C:\Windows\SysWOW64")]
+		public void TestGetRelativePathWindows(string expectedRelPath, string toPath, string fromPath) // swapped here instead of in data
+		{
+			PlatformTestUtils.OnlyRunOnWindows();
+
+			Assert.AreEqual(expectedRelPath, PE.GetRelativePath(fromPath: fromPath, toPath: toPath));
+		}
+
+		[TestMethod]
+		[DataRow(@".\SysWOW64", @"C:\Windows\SysWOW64", @"C:\Windows")]
+		[DataRow(@".", @"C:\Windows", @"C:\Windows")]
+		[DataRow(@"C:\Windows", @"C:\Windows", @"C:\Windows\SysWOW64")] // not `..`
+		[DataRow(@"C:\Windows\System32", @"C:\Windows\System32", @"C:\Windows\SysWOW64")] // not `..\System32`
+		[DataRow(@"C:\Program Files", @"C:\Program Files", @"C:\Windows\SysWOW64")] // not `..\..\Program Files`
+		public void TestMakeRelativeToWindows(string expectedRelPath, string absolutePath, string basePath)
+		{
+			PlatformTestUtils.OnlyRunOnWindows();
+
+			Assert.AreEqual(expectedRelPath, absolutePath.MakeRelativeTo(basePath));
+		}
+	}
+}
diff --git a/src/BizHawk.Tests/PlatformTests/PlatformTestUtils.cs b/src/BizHawk.Tests/PlatformTests/PlatformTestUtils.cs
new file mode 100644
index 0000000000..8e394d0422
--- /dev/null
+++ b/src/BizHawk.Tests/PlatformTests/PlatformTestUtils.cs
@@ -0,0 +1,24 @@
+using BizHawk.Common;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace BizHawk.Tests
+{
+	public static class PlatformTestUtils
+	{
+		public static void OnlyRunOnRealUnix()
+		{
+			if (!OSTailoredCode.IsUnixHost || OSTailoredCode.IsWSL) Assert.Inconclusive();
+		}
+
+		public static void OnlyRunOnUnix()
+		{
+			if (!OSTailoredCode.IsUnixHost) Assert.Inconclusive();
+		}
+
+		public static void OnlyRunOnWindows()
+		{
+			if (OSTailoredCode.IsUnixHost) Assert.Inconclusive();
+		}
+	}
+}