 
     }
 
-    /// An enumeration representing the kNumberSpacingType features.
+    /// A feature provider for changing the number spacing, also known as figure spacing.
     public enum NumberSpacing: FontFeatureProvider {
 
+        /// Monospaced numbers, also known as tabular figures, each take up the
+        /// same amount of horizontal space, meaning that different numbers will
+        /// line up when arranged in columns.
         case monospaced
+
+        /// Proportionally spaced numbers, also known as proprotional figures,
+        /// are of variable width. This makes them look better in most cases,
+        /// but they should be avoided when numbers need to line up in columns.
         case proportional
 
         public func featureSettings() -> [(type: Int, selector: Int)] {


    }

    /// An enumeration representing the kVerticalPositionType features
    public enum VerticalPosition: FontFeatureProvider {

        case normal
        case superscript
        case `subscript`
        case ordinals
        case scientificInferiors

        public func featureSettings() -> [(type: Int, selector: Int)] {

        }
    }

    /// A feature provider for changing the number case, also known as figure style.
    public enum NumberCase: FontFeatureProvider {

        /// Uppercase numbers, also known as lining figures, are the same height
        /// as uppercase letters, and they do not extend below the baseline.
        case upper

        /// Lowercase numbers, also known as oldstyle figures, are similar in
        /// size and visual weight to lowercase letters, allowing them to
        /// blend in better in a block of text. They may have descenders
        /// which drop below the typographic baseline.
        case lower

        public func featureSettings() -> [(type: Int, selector: Int)] {
