UnityAlignAtFeature.cs

        // assembly == the gameobject root to move
        // feature == the specific place on the assembly to align with station
        // station == the reference to which the assembly feature will align
        // flip == an additional rotation applied to the assembly pivoting
        //         around the feature (e.g., 180º Y flip)
        //
        public static void AlignAtFeature(
            Transform assembly, Transform feature, Transform station,
            Quaternion flip)
        {
            Assert.IsTrue(assembly != null);
            Assert.IsTrue(feature != null);
            Assert.IsTrue(feature.IsChildOf(assembly));
            Assert.IsTrue(station != null);
 
            assembly.rotation =
                station.rotation *
                Quaternion.Inverse(
                    Quaternion.Inverse(assembly.rotation) *
                        feature.rotation) *
                flip;
 
            assembly.position =
                station.position +
                (assembly.position - feature.position);
        }

Every now and then I want to align some object to another object, but relative to a specific part. Spaceship to dock, at the spaceship's airlock door. Road tile to road tile, at the painted stripe. Inventory item to hand bone, at the handle. And so on. It's a very regular recurring problem, and thinking about Quaternions and inverse transforms always takes a minute.