diff --git a/base/traits.hpp b/base/traits.hpp
index bcf95c0cd5..d62399bdc5 100644
--- a/base/traits.hpp
+++ b/base/traits.hpp
@@ -59,6 +59,11 @@ using internal::all_different_v;
 template<template<typename...> typename T, typename U>
 inline constexpr bool is_instance_of_v = internal::is_instance_of<T, U>::value;
 
+// Note the argument inversion so that we can use instance_of<T> in a requires
+// clause.
+template<typename U, template<typename...> typename T>
+concept instance_of = is_instance_of_v<T, U>;
+
 // True if and only if T and U are the same template.
 template<template<typename...> typename T, template<typename...> typename U>
 inline constexpr bool is_same_template_v =
diff --git a/geometry/grassmann.hpp b/geometry/grassmann.hpp
index 40b6b01096..df04768e80 100644
--- a/geometry/grassmann.hpp
+++ b/geometry/grassmann.hpp
@@ -29,10 +29,10 @@ using namespace principia::quantities::_quantities;
 // ⋀ⁿ Scalar³. Do not use this type for |rank == 0| (scalars), use the |Scalar|
 // type directly instead.
 // |Frame| represents a reference frame together with an orthonormal basis.
-template<typename Scalar, typename Frame, int rank>
+template<quantity Scalar, typename Frame, int rank>
 class Multivector;
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 class Multivector<Scalar, Frame, 1> final {
  public:
   static constexpr int dimension = 3;
@@ -66,11 +66,11 @@ class Multivector<Scalar, Frame, 1> final {
  private:
   R3Element<Scalar> coordinates_;
 
-  template<typename S, typename F, int r>
+  template<quantity S, typename F, int r>
   friend class Multivector;
 };
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 class Multivector<Scalar, Frame, 2> final {
  public:
   static constexpr int dimension = 3;
@@ -103,11 +103,11 @@ class Multivector<Scalar, Frame, 2> final {
  private:
   R3Element<Scalar> coordinates_;
 
-  template<typename S, typename F, int r>
+  template<quantity S, typename F, int r>
   friend class Multivector;
 };
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 class Multivector<Scalar, Frame, 3> final {
  public:
   static constexpr int dimension = 1;
@@ -137,11 +137,11 @@ class Multivector<Scalar, Frame, 3> final {
   Scalar coordinates_;
 };
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 using Vector = Multivector<Scalar, Frame, 1>;
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 using Bivector = Multivector<Scalar, Frame, 2>;
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 using Trivector = Multivector<Scalar, Frame, 3>;
 
 template<typename LScalar, typename RScalar, typename Frame>
diff --git a/geometry/grassmann_body.hpp b/geometry/grassmann_body.hpp
index 4843c56c65..11ae64c1ce 100644
--- a/geometry/grassmann_body.hpp
+++ b/geometry/grassmann_body.hpp
@@ -17,149 +17,149 @@ using namespace principia::base::_not_constructible;
 using namespace principia::geometry::_sign;
 using namespace principia::quantities::_elementary_functions;
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 1>::Multivector(R3Element<Scalar> const& coordinates)
     : coordinates_(coordinates) {}
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 2>::Multivector(R3Element<Scalar> const& coordinates)
     : coordinates_(coordinates) {}
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 3>::Multivector(Scalar const& coordinates)
     : coordinates_(coordinates) {}
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 1>& Multivector<Scalar, Frame, 1>::operator+=(
     Multivector const& right) {
   coordinates_ += right.coordinates_;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 1>& Multivector<Scalar, Frame, 1>::operator-=(
     Multivector const& right) {
   coordinates_ -= right.coordinates_;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 1>& Multivector<Scalar, Frame, 1>::operator*=(
     double const right) {
   coordinates_ *= right;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 1>& Multivector<Scalar, Frame, 1>::operator/=(
     double const right) {
   coordinates_ /= right;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 2>& Multivector<Scalar, Frame, 2>::operator+=(
     Multivector const& right) {
   coordinates_ += right.coordinates_;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 2>& Multivector<Scalar, Frame, 2>::operator-=(
     Multivector const& right) {
   coordinates_ -= right.coordinates_;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 2>& Multivector<Scalar, Frame, 2>::operator*=(
     double const right) {
   coordinates_ *= right;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 2>& Multivector<Scalar, Frame, 2>::operator/=(
     double const right) {
   coordinates_ /= right;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 3>& Multivector<Scalar, Frame, 3>::operator+=(
     Multivector const& right) {
   coordinates_ += right.coordinates_;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 3>& Multivector<Scalar, Frame, 3>::operator-=(
     Multivector const& right) {
   coordinates_ -= right.coordinates_;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 3>& Multivector<Scalar, Frame, 3>::operator*=(
     double const right) {
   coordinates_ *= right;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 3>& Multivector<Scalar, Frame, 3>::operator/=(
     double const right) {
   coordinates_ /= right;
   return *this;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 R3Element<Scalar> const& Multivector<Scalar, Frame, 1>::coordinates() const {
   return coordinates_;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 R3Element<Scalar> const& Multivector<Scalar, Frame, 2>::coordinates() const {
   return coordinates_;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Scalar const& Multivector<Scalar, Frame, 3>::coordinates() const {
   return coordinates_;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Scalar Multivector<Scalar, Frame, 1>::Norm() const {
   return coordinates_.Norm();
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Scalar Multivector<Scalar, Frame, 2>::Norm() const {
   return coordinates_.Norm();
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Scalar Multivector<Scalar, Frame, 3>::Norm() const {
   // When |Scalar| is double, ADL will not find |Abs|.
   return Abs(coordinates_);
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Square<Scalar> Multivector<Scalar, Frame, 1>::Norm²() const {
   return coordinates_.Norm²();
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Square<Scalar> Multivector<Scalar, Frame, 2>::Norm²() const {
   return coordinates_.Norm²();
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Square<Scalar> Multivector<Scalar, Frame, 3>::Norm²() const {
   return coordinates_ * coordinates_;
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 template<typename S>
 Multivector<Scalar, Frame, 1>
 Multivector<Scalar, Frame, 1>::OrthogonalizationAgainst(
@@ -168,7 +168,7 @@ Multivector<Scalar, Frame, 1>::OrthogonalizationAgainst(
       coordinates_.OrthogonalizationAgainst(multivector.coordinates_));
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 template<typename S>
 Multivector<Scalar, Frame, 2>
 Multivector<Scalar, Frame, 2>::OrthogonalizationAgainst(
@@ -177,28 +177,28 @@ Multivector<Scalar, Frame, 2>::OrthogonalizationAgainst(
       coordinates_.OrthogonalizationAgainst(multivector.coordinates_));
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 void Multivector<Scalar, Frame, 1>::WriteToMessage(
       not_null<serialization::Multivector*> const message) const {
   Frame::WriteToMessage(message->mutable_frame());
   coordinates_.WriteToMessage(message->mutable_vector());
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 void Multivector<Scalar, Frame, 2>::WriteToMessage(
       not_null<serialization::Multivector*> const message) const {
   Frame::WriteToMessage(message->mutable_frame());
   coordinates_.WriteToMessage(message->mutable_bivector());
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 void Multivector<Scalar, Frame, 3>::WriteToMessage(
       not_null<serialization::Multivector*> const message) const {
   Frame::WriteToMessage(message->mutable_frame());
   coordinates_.WriteToMessage(message->mutable_trivector());
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 1> Multivector<Scalar, Frame, 1>::ReadFromMessage(
     serialization::Multivector const& message)
   requires serializable<Frame> {
@@ -207,7 +207,7 @@ Multivector<Scalar, Frame, 1> Multivector<Scalar, Frame, 1>::ReadFromMessage(
   return Multivector(R3Element<Scalar>::ReadFromMessage(message.vector()));
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 2> Multivector<Scalar, Frame, 2>::ReadFromMessage(
     serialization::Multivector const& message)
   requires serializable<Frame> {
@@ -216,7 +216,7 @@ Multivector<Scalar, Frame, 2> Multivector<Scalar, Frame, 2>::ReadFromMessage(
   return Multivector(R3Element<Scalar>::ReadFromMessage(message.bivector()));
 }
 
-template<typename Scalar, typename Frame>
+template<quantity Scalar, typename Frame>
 Multivector<Scalar, Frame, 3> Multivector<Scalar, Frame, 3>::ReadFromMessage(
     serialization::Multivector const& message)
   requires serializable<Frame> {
diff --git a/geometry/grassmann_test.cpp b/geometry/grassmann_test.cpp
index a0016b8465..19453b4fe0 100644
--- a/geometry/grassmann_test.cpp
+++ b/geometry/grassmann_test.cpp
@@ -69,6 +69,7 @@ class GrassmannTest : public testing::Test {
                       Inertial,
                       Handedness::Right,
                       serialization::Frame::TEST>;
+  Vector<World, Acceleration> γ;
 
   R3Element<Length> const null_displacement_ = {0 * Metre,
                                                 0 * Metre,
diff --git a/ksp_plugin_adapter_stub/ksp_plugin_adapter_stub.dll b/ksp_plugin_adapter_stub/ksp_plugin_adapter_stub.dll
index f9798413cf..8449cc1854 100644
Binary files a/ksp_plugin_adapter_stub/ksp_plugin_adapter_stub.dll and b/ksp_plugin_adapter_stub/ksp_plugin_adapter_stub.dll differ
diff --git a/ksp_plugin_adapter_stub/ksp_plugin_adapter_stub.pdb b/ksp_plugin_adapter_stub/ksp_plugin_adapter_stub.pdb
index 706257f21b..707969b1cb 100644
Binary files a/ksp_plugin_adapter_stub/ksp_plugin_adapter_stub.pdb and b/ksp_plugin_adapter_stub/ksp_plugin_adapter_stub.pdb differ
diff --git a/quantities/concepts.hpp b/quantities/concepts.hpp
index 2b46b19bbf..d88785e2af 100644
--- a/quantities/concepts.hpp
+++ b/quantities/concepts.hpp
@@ -49,18 +49,21 @@ concept affine_space = requires(A x, A y) {
 template<typename V>
 concept real_affine_space = affine_space<V, double>;
 
+template<typename T>
+concept quantity = instance_of<T, Quantity> || std::same_as<T, double>;
+
 template<typename T>
 concept convertible_to_quantity =
-    std::convertible_to<T, double> ||
-    is_instance_of_v<Quantity, std::remove_cvref_t<T>>;
+    std::convertible_to<T, double> || quantity<std::remove_cvref_t<T>>;
 
 }  // namespace internal
 
 using internal::additive_group;
 using internal::affine_space;
+using internal::convertible_to_quantity;
+using internal::quantity;
 using internal::real_affine_space;
 using internal::real_vector_space;
-using internal::convertible_to_quantity;
 using internal::vector_space;
 
 }  // namespace _concepts