diff --git a/include/beman/inplace_vector/inplace_vector.hpp b/include/beman/inplace_vector/inplace_vector.hpp
index 320cfd1..8153afb 100644
--- a/include/beman/inplace_vector/inplace_vector.hpp
+++ b/include/beman/inplace_vector/inplace_vector.hpp
@@ -254,6 +254,7 @@ Software.
  */
 #include <algorithm> // for rotate...
 #include <array>
+#include <compare>
 #include <concepts>   // for lots...
 #include <cstddef>    // for size_t
 #include <cstdint>    // for fixed-width integer types
@@ -309,6 +310,11 @@ concept container_compatible_range =
 template <typename T, std::size_t N>
 concept satify_constexpr = N == 0 || std::is_trivial_v<T>;
 
+template <typename T>
+concept lessthan_comparable = requires(const T &a, const T &b) {
+  { a < b } -> std::convertible_to<bool>;
+};
+
 } // namespace beman::details::inplace_vector
 
 // Types implementing the `inplace_vector`'s storage
@@ -1001,27 +1007,25 @@ struct inplace_vector
     insert_range(begin(), il);
   }
 
-  constexpr friend int /*synth-three-way-result<T>*/
-  operator<=>(const inplace_vector & x, const inplace_vector & y) {
-    if (x.size() < y.size())
-      return -1;
-    if (x.size() > y.size())
-      return +1;
-
-    bool all_equal = true;
-    bool all_less = true;
-    for (size_type i = 0; i < x.size(); ++i) {
-      if (x[i] < y[i])
-        all_equal = false;
-      if (x[i] == y[i])
-        all_less = false;
+  constexpr friend auto operator<=>(const inplace_vector &x,
+                                    const inplace_vector &y)
+    requires(beman::details::inplace_vector::lessthan_comparable<T>)
+  {
+    if constexpr (std::three_way_comparable<T>) {
+      return std::lexicographical_compare_three_way(x.begin(), x.end(),
+                                                    y.begin(), y.end());
+    } else {
+      const auto sz = std::min(x.size(), y.size());
+      for (std::size_t i = 0; i < sz; ++i) {
+        if (x[i] < y[i])
+          return std::strong_ordering::less;
+        if (y[i] < x[i])
+          return std::strong_ordering::greater;
+        // [container.opt.reqmts] < must be total ordering relationship
+      }
+
+      return x.size() <=> y.size();
     }
-
-    if (all_equal)
-      return 0;
-    if (all_less)
-      return -1;
-    return 1;
   }
 };
 
diff --git a/tests/beman/inplace_vector/CMakeLists.txt b/tests/beman/inplace_vector/CMakeLists.txt
index e0a01b4..a746209 100644
--- a/tests/beman/inplace_vector/CMakeLists.txt
+++ b/tests/beman/inplace_vector/CMakeLists.txt
@@ -37,6 +37,7 @@ endfunction()
 # Tests for official specs
 add_gtest(container_requirements)
 add_gtest(triviality)
+add_gtest(compare)
 add_gtest(constructors)
 add_gtest(size_n_data)
 add_gtest(erasure)
diff --git a/tests/beman/inplace_vector/compare.test.cpp b/tests/beman/inplace_vector/compare.test.cpp
new file mode 100644
index 0000000..cdaa458
--- /dev/null
+++ b/tests/beman/inplace_vector/compare.test.cpp
@@ -0,0 +1,326 @@
+#include <beman/inplace_vector/inplace_vector.hpp>
+#include <gtest/gtest.h>
+
+#include <cmath>
+#include <compare>
+
+using namespace beman;
+
+template <class T>
+concept has_threeway = requires(const T &t) {
+  { t <=> t };
+};
+
+template <class T>
+concept lessthan_comparable =
+    beman::details::inplace_vector::lessthan_comparable<T>;
+
+template <typename T> struct vec_list {
+  T empty;
+  T base;            // base
+  T copy;            // identical to base
+  T greater;         // greater number of elements
+  T lesser;          // lesser number of elements
+  T bigger;          // bigger value of the elements
+  T smaller;         // smaller value of the elements
+  T greater_smaller; // greater number of elements but smaller values
+  T lesser_bigger;   // lesser number of elements but bigger values
+};
+
+template <typename T> static void runtests(vec_list<T> &list) {
+
+  static_assert(std::three_way_comparable<T> || lessthan_comparable<T>);
+
+  // if T::value_type is threewaycomparable with ordering X then T must also
+  // be comparable with ordering X
+
+  using VT = typename T::value_type;
+
+  if constexpr (std::three_way_comparable<VT, std::strong_ordering>)
+    static_assert(std::three_way_comparable<T, std::strong_ordering>);
+
+  if constexpr (std::three_way_comparable<VT, std::weak_ordering>)
+    static_assert(std::three_way_comparable<T, std::weak_ordering>);
+
+  if constexpr (std::three_way_comparable<VT, std::partial_ordering>)
+    static_assert(std::three_way_comparable<T, std::partial_ordering>);
+
+  if constexpr (std::equality_comparable<VT>) {
+    EXPECT_TRUE(list.empty == list.empty);
+    EXPECT_TRUE(list.empty != list.base);
+  }
+
+  EXPECT_TRUE((list.base <=> list.copy) == 0);
+  EXPECT_TRUE((list.base <=> list.greater) < 0);
+  EXPECT_TRUE((list.base <=> list.lesser) > 0);
+
+  EXPECT_TRUE((list.base <=> list.bigger) < 0);
+  EXPECT_TRUE((list.base <=> list.smaller) > 0);
+
+  EXPECT_TRUE((list.base <=> list.greater_smaller) < 0);
+  EXPECT_TRUE((list.base <=> list.lesser_bigger) > 0);
+
+  if constexpr (std::equality_comparable<VT>) {
+    EXPECT_TRUE(list.base == list.copy);
+    EXPECT_TRUE(list.base != list.greater);
+    EXPECT_TRUE(list.base != list.lesser);
+  }
+  EXPECT_TRUE(list.base <= list.copy);
+  EXPECT_TRUE(list.base >= list.copy);
+  EXPECT_TRUE(list.base < list.greater);
+  EXPECT_TRUE(list.base <= list.greater);
+  EXPECT_TRUE(list.base > list.lesser);
+  EXPECT_TRUE(list.base >= list.lesser);
+
+  if constexpr (std::equality_comparable<VT>) {
+    EXPECT_TRUE(list.copy == list.base);
+    EXPECT_TRUE(list.copy != list.greater);
+    EXPECT_TRUE(list.copy != list.lesser);
+  }
+  EXPECT_TRUE(list.copy <= list.base);
+  EXPECT_TRUE(list.copy >= list.base);
+  EXPECT_TRUE(list.greater > list.base);
+  EXPECT_TRUE(list.greater >= list.base);
+  EXPECT_TRUE(list.lesser < list.base);
+  EXPECT_TRUE(list.lesser <= list.base);
+};
+
+TEST(Compare, threeway_int) {
+  vec_list<inplace_vector<int, 4>> list{
+      .empty{},
+      .base{1, 2, 3},
+      .copy{1, 2, 3},
+      .greater{4, 5, 6},
+      .lesser{0, 0, 0},
+      .bigger{1, 2, 3, 0},
+      .smaller{1, 2},
+      .greater_smaller{2, 2},
+      .lesser_bigger{0, 2, 3, 4},
+  };
+
+  runtests(list);
+}
+
+TEST(Compare, threeway_float) {
+  vec_list<inplace_vector<float, 4>> list{
+      .empty{},
+      .base{1.0f, 2.0f, 3.0f},
+      .copy{1.0f, 2.0f, 3.0f},
+      .greater{4.0f, 5.0f, 6.0f},
+      .lesser{0.0f, 0.0f, 0.0f},
+      .bigger{1.0f, 2.0f, 3.0f, 0.0f},
+      .smaller{1.0f, 2.0f},
+      .greater_smaller{2.0f, 2.0f},
+      .lesser_bigger{0.0f, 2.0f, 3.0f, 4.0f},
+  };
+
+  runtests(list);
+
+  // compare unorderable values
+
+  EXPECT_EQ(std::nanf("") <=> std::nanf(""), std::partial_ordering::unordered);
+  EXPECT_FALSE(std::nanf("") == std::nanf(""));
+  EXPECT_FALSE(std::nanf("") < std::nanf(""));
+  EXPECT_FALSE(std::nanf("") > std::nanf(""));
+  EXPECT_FALSE(std::nanf("") >= std::nanf(""));
+  EXPECT_FALSE(std::nanf("") <= std::nanf(""));
+
+  inplace_vector<float, 4> vnan{std::nanf("")};
+  inplace_vector<float, 4> vnan2{std::nanf("")};
+
+  EXPECT_EQ(vnan <=> vnan2, std::partial_ordering::unordered);
+  EXPECT_FALSE(vnan == vnan2);
+  EXPECT_FALSE(vnan < vnan2);
+  EXPECT_FALSE(vnan > vnan2);
+  EXPECT_FALSE(vnan >= vnan2);
+  EXPECT_FALSE(vnan <= vnan2);
+}
+
+TEST(Compare, threeway_comparable1) {
+  struct comparable1 {
+    int a;
+    int b;
+    constexpr auto operator<=>(const comparable1 &) const = default;
+  };
+
+  static_assert(std::three_way_comparable<comparable1>);
+  static_assert(has_threeway<comparable1>);
+  static_assert(std::three_way_comparable<inplace_vector<comparable1, 4>>);
+  static_assert(has_threeway<inplace_vector<comparable1, 4>>);
+
+  vec_list<inplace_vector<comparable1, 4>> list{
+      .empty{},
+      .base{{1, 2}, {3, 4}},
+      .copy{{1, 2}, {3, 4}},
+      .greater{{5, 6}, {7, 8}},
+      .lesser{{0, 0}, {0, 0}},
+      .bigger{{1, 2}, {3, 4}, {5, 6}},
+      .smaller{{1, 2}},
+      .greater_smaller{{2, 2}, {3, 3}},
+      .lesser_bigger{{0, 2}, {3, 3}, {4, 4}},
+  };
+
+  runtests(list);
+}
+
+TEST(Compare, threeway_comparable2) {
+
+  struct comparable2 {
+    int a;
+    int b;
+    constexpr bool operator==(const comparable2 &) const = delete;
+    constexpr bool operator<(const comparable2 &other) const {
+      return a < other.a || (a == other.a && b < other.b);
+    };
+  };
+
+  static_assert(!std::three_way_comparable<comparable2>);
+  static_assert(!has_threeway<comparable2>);
+  static_assert(lessthan_comparable<comparable2>);
+  static_assert(std::three_way_comparable<inplace_vector<comparable2, 4>>);
+  static_assert(has_threeway<inplace_vector<comparable2, 4>>);
+
+  vec_list<inplace_vector<comparable2, 4>> list{
+      .empty{},
+      .base{{1, 2}, {3, 4}},
+      .copy{{1, 2}, {3, 4}},
+      .greater{{5, 6}, {7, 8}},
+      .lesser{{0, 0}, {0, 0}},
+      .bigger{{1, 2}, {3, 4}, {5, 6}},
+      .smaller{{1, 2}},
+      .greater_smaller{{2, 2}, {3, 3}},
+      .lesser_bigger{{0, 2}, {3, 3}, {4, 4}},
+  };
+
+  runtests(list);
+}
+
+TEST(Compare, threeway_strong_ordering) {
+
+  struct weaktype {
+    int a;
+    constexpr std::strong_ordering
+    operator<=>(const weaktype &other) const = default;
+  };
+
+  using T = weaktype;
+
+  vec_list<inplace_vector<weaktype, 4>> list{
+      .empty{},
+      .base{T{1}, T{2}, T{3}},
+      .copy{T{1}, T{2}, T{3}},
+      .greater{T{4}, T{5}, T{6}},
+      .lesser{T{0}, T{0}, T{0}},
+      .bigger{T{1}, T{2}, T{3}, T{0}},
+      .smaller{T{1}, T{2}},
+      .greater_smaller{T{2}, T{2}},
+      .lesser_bigger{T{0}, T{2}, T{3}, T{4}},
+  };
+
+  runtests(list);
+}
+
+TEST(Compare, threeway_weak_ordering) {
+
+  struct weaktype {
+    int a;
+    constexpr std::weak_ordering
+    operator<=>(const weaktype &other) const = default;
+  };
+
+  using T = weaktype;
+
+  vec_list<inplace_vector<weaktype, 4>> list{
+      .empty{},
+      .base{T{1}, T{2}, T{3}},
+      .copy{T{1}, T{2}, T{3}},
+      .greater{T{4}, T{5}, T{6}},
+      .lesser{T{0}, T{0}, T{0}},
+      .bigger{T{1}, T{2}, T{3}, T{0}},
+      .smaller{T{1}, T{2}},
+      .greater_smaller{T{2}, T{2}},
+      .lesser_bigger{T{0}, T{2}, T{3}, T{4}},
+  };
+
+  runtests(list);
+}
+
+TEST(Compare, threeway_partial_ordering) {
+
+  struct custom {
+    int a;
+    constexpr auto operator<=>(const custom &other) const {
+      if (a == -1 && other.a == -1)
+        return std::partial_ordering::unordered;
+      return std::partial_ordering(a <=> other.a);
+    }
+
+    constexpr bool operator==(const custom &other) const {
+      if (a == -1 && other.a == -1)
+        return false;
+      return a == other.a;
+    }
+  };
+
+  using T = custom;
+
+  vec_list<inplace_vector<custom, 4>> list{
+      .empty{},
+      .base{T{1}, T{2}, T{3}},
+      .copy{T{1}, T{2}, T{3}},
+      .greater{T{4}, T{5}, T{6}},
+      .lesser{T{0}, T{0}, T{0}},
+      .bigger{T{1}, T{2}, T{3}, T{0}},
+      .smaller{T{1}, T{2}},
+      .greater_smaller{T{2}, T{2}},
+      .lesser_bigger{T{0}, T{2}, T{3}, T{4}},
+  };
+
+  runtests(list);
+
+  T t1{-1};
+  T t2 = t1;
+  EXPECT_EQ(t1 <=> t2, std::partial_ordering::unordered);
+
+  inplace_vector<T, 4> v1{t1};
+  inplace_vector<T, 4> v2{t2};
+
+  EXPECT_EQ(v1 <=> v2, std::partial_ordering::unordered);
+  EXPECT_FALSE(v1 == v2);
+  EXPECT_FALSE(v1 < v2);
+  EXPECT_FALSE(v1 > v2);
+  EXPECT_FALSE(v1 >= v2);
+  EXPECT_FALSE(v1 <= v2);
+}
+
+TEST(Compare, threeway_uncomparable) {
+
+  struct uncomparable1 {
+    int a;
+  };
+
+  static_assert(!std::three_way_comparable<uncomparable1>);
+  static_assert(!has_threeway<uncomparable1>);
+  static_assert(!std::three_way_comparable<inplace_vector<uncomparable1, 4>>);
+  static_assert(!has_threeway<inplace_vector<uncomparable1, 4>>);
+
+  struct uncomparable2 {
+    int a;
+    constexpr bool operator==(const uncomparable2 &) const = default;
+  };
+
+  static_assert(!std::three_way_comparable<uncomparable2>);
+  static_assert(!has_threeway<uncomparable2>);
+  static_assert(!std::three_way_comparable<inplace_vector<uncomparable2, 4>>);
+  static_assert(!has_threeway<inplace_vector<uncomparable2, 4>>);
+
+  struct uncomparable3 {
+    int a;
+    constexpr auto operator<=>(const uncomparable3 &) const = delete;
+  };
+
+  static_assert(!std::three_way_comparable<uncomparable3>);
+  static_assert(!has_threeway<uncomparable3>);
+  static_assert(!std::three_way_comparable<inplace_vector<uncomparable3, 4>>);
+  static_assert(!has_threeway<inplace_vector<uncomparable3, 4>>);
+}