diff --git a/.github/workflows/ci.bazelrc b/.github/workflows/ci.bazelrc
index 6c7541e..542a28b 100644
--- a/.github/workflows/ci.bazelrc
+++ b/.github/workflows/ci.bazelrc
@@ -5,7 +5,6 @@ build --show_timestamps
 build --announce_rc
 build --color=yes
 build --terminal_columns=120
-build --remote_download_minimal
 build --curses=no
 
 test --test_output=errors
diff --git a/BUILD.bazel b/BUILD.bazel
index e7de441..7a39643 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -16,7 +16,9 @@ filegroup(
 
 cc_library(
     name = "geometry",
-    srcs = [],
+    srcs = [
+        "geometry/src/algebra.hpp",
+    ],
     hdrs = ["geometry/geometry.hpp"],
     visibility = ["//visibility:public"],
 )
diff --git a/geometry/geometry.hpp b/geometry/geometry.hpp
index 9ea2134..27fb55b 100644
--- a/geometry/geometry.hpp
+++ b/geometry/geometry.hpp
@@ -1,10 +1,3 @@
 #pragma once
 
-namespace geometry {
-
-constexpr auto dummy_function()
-{
-  return 0;
-}
-
-}  // namespace geometry
+#include "geometry/src/algebra.hpp"
diff --git a/geometry/src/algebra.hpp b/geometry/src/algebra.hpp
new file mode 100644
index 0000000..fa79a2d
--- /dev/null
+++ b/geometry/src/algebra.hpp
@@ -0,0 +1,98 @@
+#pragma once
+
+#include <cstddef>
+#include <type_traits>
+
+namespace geometry {
+
+template <class S, std::size_t N>
+struct algebra
+{
+  using scalar_type = S;
+
+  template <std::size_t... Is>
+  struct blade
+  {
+    static_assert(sizeof...(Is) <= N + 1, "number of `Is` exceeds rank");
+    static_assert(((Is <= N) and ...), "`Is` exceeds rank");
+    // TODO check Is.. values are unique
+
+    using algebra_type = algebra;
+    using scalar_type = typename algebra::scalar_type;
+
+    static constexpr auto grade = sizeof...(Is);
+
+    scalar_type coefficient{};
+
+    blade() = default;
+
+    template <std::size_t M = grade, std::enable_if_t<M == 0, bool> = true>
+    constexpr blade(scalar_type a) : coefficient{a}
+    {}
+    template <std::size_t M = grade, std::enable_if_t<M != 0, bool> = true>
+    constexpr explicit blade(scalar_type a) : coefficient{a}
+    {}
+
+    [[nodiscard]]
+    friend constexpr auto
+    operator==(blade x, blade y) noexcept -> bool
+    {
+      return x.coefficient == y.coefficient;
+    }
+    [[nodiscard]]
+    friend constexpr auto
+    operator!=(blade x, blade y) noexcept -> bool
+    {
+      return x.coefficient != y.coefficient;
+    }
+    [[nodiscard]]
+    friend constexpr auto
+    operator<(blade x, blade y) noexcept -> bool
+    {
+      return x.coefficient < y.coefficient;
+    }
+    [[nodiscard]]
+    friend constexpr auto
+    operator>(blade x, blade y) noexcept -> bool
+    {
+      return x.coefficient > y.coefficient;
+    }
+    [[nodiscard]]
+    friend constexpr auto
+    operator<=(blade x, blade y) noexcept -> bool
+    {
+      return x.coefficient <= y.coefficient;
+    }
+    [[nodiscard]]
+    friend constexpr auto
+    operator>=(blade x, blade y) noexcept -> bool
+    {
+      return x.coefficient >= y.coefficient;
+    }
+
+    [[nodiscard]]
+    friend constexpr auto
+    operator-(blade x) -> blade
+    {
+      return blade{-x.coefficient};
+    }
+
+    [[nodiscard]]
+    friend constexpr auto
+    operator*(scalar_type a, blade x) -> blade
+    {
+      return blade{a * x.coefficient};
+    }
+    [[nodiscard]]
+    friend constexpr auto
+    operator*(blade x, scalar_type a) -> blade
+    {
+      return a * x;
+    }
+  };
+
+  template <std::size_t... Is>
+  static constexpr auto e = blade<Is...>{scalar_type{1}};
+};
+
+}  // namespace geometry
diff --git a/test/BUILD.bazel b/test/BUILD.bazel
index 7e5f5eb..7009c30 100644
--- a/test/BUILD.bazel
+++ b/test/BUILD.bazel
@@ -1,8 +1,8 @@
 load("@rules_cc//cc:defs.bzl", "cc_test")
 
 cc_test(
-    name = "dummy_test",
-    srcs = ["dummy_test.cpp"],
+    name = "algebra_test",
+    srcs = ["algebra_test.cpp"],
     deps = [
         "//:geometry",
         "@skytest",
diff --git a/test/algebra_test.cpp b/test/algebra_test.cpp
new file mode 100644
index 0000000..8283ea2
--- /dev/null
+++ b/test/algebra_test.cpp
@@ -0,0 +1,55 @@
+#include "geometry/geometry.hpp"
+#include "skytest/skytest.hpp"
+
+#include <cstddef>
+#include <tuple>
+#include <type_traits>
+
+template <std::size_t... Is>
+constexpr auto e = ::geometry::algebra<double, 2>::e<Is...>;
+
+constexpr auto unit_blades = std::tuple{
+    ::geometry::algebra<double, 2>::e<>,
+    ::geometry::algebra<double, 2>::e<0>,
+    ::geometry::algebra<double, 2>::e<1>,
+    ::geometry::algebra<double, 2>::e<2>,
+    ::geometry::algebra<double, 2>::e<0, 1>,
+    ::geometry::algebra<double, 2>::e<0, 2>,
+    ::geometry::algebra<double, 2>::e<1, 2>,
+    ::geometry::algebra<double, 2>::e<0, 1, 2>};
+
+auto main() -> int
+{
+  using namespace ::skytest::literals;
+  using ::skytest::eq;
+  using ::skytest::expect;
+  using ::skytest::param_ref;
+
+  "0-blade implicitly constructible"_ctest = [] {
+    return expect(
+        eq(e<>, e<>) and    //
+        eq(0, 0 * e<>) and  //
+        eq(1, 1 * e<>) and  //
+        eq(-1, -e<>));
+  };
+
+  "grade 1+ blade explicitly constructible"_ctest * param_ref<unit_blades> =  //
+      [](auto ei) {
+        return expect(
+            ei.grade == 0 or  //
+            not std::is_convertible_v<double, decltype(ei)>);
+      };
+
+  "grade 0+ blades comparable"_ctest * param_ref<unit_blades> =  //
+      [](auto ei) {
+        using namespace ::skytest;
+
+        return expect(
+            eq(ei, ei) and      //
+            ne(0 * ei, ei) and  //
+            lt(0 * ei, ei) and  //
+            le(0 * ei, ei) and  //
+            gt(ei, 0 * ei) and  //
+            ge(ei, 0 * ei));
+      };
+}
diff --git a/test/dummy_test.cpp b/test/dummy_test.cpp
deleted file mode 100644
index 16b196e..0000000
--- a/test/dummy_test.cpp
+++ /dev/null
@@ -1,11 +0,0 @@
-#include "geometry/geometry.hpp"
-#include "skytest/skytest.hpp"
-
-auto main() -> int
-{
-  using namespace ::skytest::literals;
-  using ::skytest::eq;
-  using ::skytest::expect;
-
-  "dummy"_ctest = [] { return expect(eq(0, ::geometry::dummy_function())); };
-}
diff --git a/tools/BUILD.bazel b/tools/BUILD.bazel
index baa187f..0d0febc 100644
--- a/tools/BUILD.bazel
+++ b/tools/BUILD.bazel
@@ -49,7 +49,7 @@ sh_binary(
 )
 
 lcov_attrs = {
-    "instrumented_targets": ["//:skytest"],
+    "instrumented_targets": ["//:geometry"],
     "test_targets": ["//test/..."],
     "coverage_opts": [
         "--combined_report=lcov",