diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0a58eb9..38103c7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -26,29 +26,32 @@ jobs:
     strategy:
       matrix:
         os: [macos-12, ubuntu-22.04]
-        # boost: [oldest, pinned, latest]
-        boost: [pinned, latest]
+        # I moved to C++ 20 2024-01-12-- the earliest version of boost that can compile
+        # under C++ 20 seems to be 1.75. That was released 2020-12-11, over three years
+        # ago so I'm comfortable calling that "oldest", now. Let's call 1.81 (one year ago)
+        # "pinned".
+        boost: [oldest, pinned, latest]
         include:
-        # - os: macos-12
-        #   boost: oldest
-        #   boost-ver: 1_63_0
-        #   boost-dl: https://boostorg.jfrog.io/artifactory/main/release/1.63.0/source/boost_1_63_0.tar.bz2
         - os: macos-12
-          boost: pinned
+          boost: oldest
           boost-ver: 1_75_0
           boost-dl: https://boostorg.jfrog.io/artifactory/main/release/1.75.0/source/boost_1_75_0.tar.bz2
+        - os: macos-12
+          boost: pinned
+          boost-ver: 1_81_0
+          boost-dl: https://boostorg.jfrog.io/artifactory/main/release/1.81.0/source/boost_1_81_0.tar.bz2
         - os: macos-12
           boost: latest
           boost-ver: 1_84_0
           boost-dl: https://boostorg.jfrog.io/artifactory/main/release/1.84.0/source/boost_1_84_0.tar.bz2
-        # - os: ubuntu-22.04
-        #   boost: oldest
-        #   boost-ver: 1_63_0
-        #   boost-dl: https://boostorg.jfrog.io/artifactory/main/release/1.63.0/source/boost_1_63_0.tar.bz2
         - os: ubuntu-22.04
-          boost: pinned
+          boost: oldest
           boost-ver: 1_75_0
           boost-dl: https://boostorg.jfrog.io/artifactory/main/release/1.75.0/source/boost_1_75_0.tar.bz2
+        - os: ubuntu-22.04
+          boost: pinned
+          boost-ver: 1_81_0
+          boost-dl: https://boostorg.jfrog.io/artifactory/main/release/1.81.0/source/boost_1_81_0.tar.bz2
         - os: ubuntu-22.04
           boost: latest
           boost-ver: latest