diff --git a/lib/ui/geometry.dart b/lib/ui/geometry.dart
index cc53d4415274a..e4bb57ec5096c 100644
--- a/lib/ui/geometry.dart
+++ b/lib/ui/geometry.dart
@@ -263,7 +263,20 @@ class Offset extends OffsetBase {
   /// Linearly interpolate between two offsets.
   ///
   /// If either offset is null, this function interpolates from [Offset.zero].
+  ///
+  /// The `t` argument represents position on the timeline, with 0.0 meaning
+  /// that the interpolation has not started, returning `a` (or something
+  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
+  /// returning `b` (or something equivalent to `b`), and values in between
+  /// meaning that the interpolation is at the relevant point on the timeline
+  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
+  /// 1.0, so negative values and values greater than 1.0 are valid (and can
+  /// easily be generated by curves such as [Curves.elasticInOut]).
+  ///
+  /// Values for `t` are usually obtained from an [Animation<double>], such as
+  /// an [AnimationController].
   static Offset lerp(Offset a, Offset b, double t) {
+    assert(t != null);
     if (a == null && b == null)
       return null;
     if (a == null)
@@ -490,7 +503,20 @@ class Size extends OffsetBase {
   /// Linearly interpolate between two sizes
   ///
   /// If either size is null, this function interpolates from [Size.zero].
+  ///
+  /// The `t` argument represents position on the timeline, with 0.0 meaning
+  /// that the interpolation has not started, returning `a` (or something
+  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
+  /// returning `b` (or something equivalent to `b`), and values in between
+  /// meaning that the interpolation is at the relevant point on the timeline
+  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
+  /// 1.0, so negative values and values greater than 1.0 are valid (and can
+  /// easily be generated by curves such as [Curves.elasticInOut]).
+  ///
+  /// Values for `t` are usually obtained from an [Animation<double>], such as
+  /// an [AnimationController].
   static Size lerp(Size a, Size b, double t) {
+    assert(t != null);
     if (a == null && b == null)
       return null;
     if (a == null)
@@ -739,7 +765,20 @@ class Rect {
   /// Linearly interpolate between two rectangles.
   ///
   /// If either rect is null, [Rect.zero] is used as a substitute.
+  ///
+  /// The `t` argument represents position on the timeline, with 0.0 meaning
+  /// that the interpolation has not started, returning `a` (or something
+  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
+  /// returning `b` (or something equivalent to `b`), and values in between
+  /// meaning that the interpolation is at the relevant point on the timeline
+  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
+  /// 1.0, so negative values and values greater than 1.0 are valid (and can
+  /// easily be generated by curves such as [Curves.elasticInOut]).
+  ///
+  /// Values for `t` are usually obtained from an [Animation<double>], such as
+  /// an [AnimationController].
   static Rect lerp(Rect a, Rect b, double t) {
+    assert(t != null);
     if (a == null && b == null)
       return null;
     if (a == null)
@@ -752,7 +791,7 @@ class Rect {
       lerpDouble(a.left, b.left, t),
       lerpDouble(a.top, b.top, t),
       lerpDouble(a.right, b.right, t),
-      lerpDouble(a.bottom, b.bottom, t)
+      lerpDouble(a.bottom, b.bottom, t),
     );
   }
 
@@ -851,7 +890,20 @@ class Radius {
   /// Linearly interpolate between two radii.
   ///
   /// If either is null, this function substitutes [Radius.zero] instead.
+  ///
+  /// The `t` argument represents position on the timeline, with 0.0 meaning
+  /// that the interpolation has not started, returning `a` (or something
+  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
+  /// returning `b` (or something equivalent to `b`), and values in between
+  /// meaning that the interpolation is at the relevant point on the timeline
+  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
+  /// 1.0, so negative values and values greater than 1.0 are valid (and can
+  /// easily be generated by curves such as [Curves.elasticInOut]).
+  ///
+  /// Values for `t` are usually obtained from an [Animation<double>], such as
+  /// an [AnimationController].
   static Radius lerp(Radius a, Radius b, double t) {
+    assert(t != null);
     if (a == null && b == null)
       return null;
     if (a == null)
@@ -862,7 +914,7 @@ class Radius {
     }
     return new Radius.elliptical(
       lerpDouble(a.x, b.x, t),
-      lerpDouble(a.y, b.y, t)
+      lerpDouble(a.y, b.y, t),
     );
   }
 
@@ -1355,7 +1407,20 @@ class RRect {
   /// Linearly interpolate between two rounded rectangles.
   ///
   /// If either is null, this function substitutes [RRect.zero] instead.
+  ///
+  /// The `t` argument represents position on the timeline, with 0.0 meaning
+  /// that the interpolation has not started, returning `a` (or something
+  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
+  /// returning `b` (or something equivalent to `b`), and values in between
+  /// meaning that the interpolation is at the relevant point on the timeline
+  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
+  /// 1.0, so negative values and values greater than 1.0 are valid (and can
+  /// easily be generated by curves such as [Curves.elasticInOut]).
+  ///
+  /// Values for `t` are usually obtained from an [Animation<double>], such as
+  /// an [AnimationController].
   static RRect lerp(RRect a, RRect b, double t) {
+    assert(t != null);
     if (a == null && b == null)
       return null;
     if (a == null) {
@@ -1371,7 +1436,7 @@ class RRect {
         b.brRadiusX * t,
         b.brRadiusY * t,
         b.blRadiusX * t,
-        b.blRadiusY * t
+        b.blRadiusY * t,
       ]);
     }
     if (b == null) {
@@ -1388,7 +1453,7 @@ class RRect {
         a.brRadiusX * k,
         a.brRadiusY * k,
         a.blRadiusX * k,
-        a.blRadiusY * k
+        a.blRadiusY * k,
       ]);
     }
     return new RRect._fromList(<double>[
@@ -1403,7 +1468,7 @@ class RRect {
       lerpDouble(a.brRadiusX, b.brRadiusX, t),
       lerpDouble(a.brRadiusY, b.brRadiusY, t),
       lerpDouble(a.blRadiusX, b.blRadiusX, t),
-      lerpDouble(a.blRadiusY, b.blRadiusY, t)
+      lerpDouble(a.blRadiusY, b.blRadiusY, t),
     ]);
   }
 
diff --git a/lib/ui/painting.dart b/lib/ui/painting.dart
index 366afb120d8fb..70e5570f37690 100644
--- a/lib/ui/painting.dart
+++ b/lib/ui/painting.dart
@@ -223,15 +223,28 @@ class Color {
 
   /// Linearly interpolate between two colors.
   ///
-  /// If either color is null, this function linearly interpolates from a
-  /// transparent instance of the other color.
-  ///
-  /// Values of `t` less that 0.0 or greater than 1.0 are supported. Each
-  /// channel will be clamped to the range 0 to 255.
-  ///
   /// This is intended to be fast but as a result may be ugly. Consider
   /// [HSVColor] or writing custom logic for interpolating colors.
+  ///
+  /// If either color is null, this function linearly interpolates from a
+  /// transparent instance of the other color. This is usually preferable to
+  /// interpolating from [Colors.transparent] (`const Color(0x00000000)`), which
+  /// is specifically transparent _black_.
+  ///
+  /// The `t` argument represents position on the timeline, with 0.0 meaning
+  /// that the interpolation has not started, returning `a` (or something
+  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
+  /// returning `b` (or something equivalent to `b`), and values in between
+  /// meaning that the interpolation is at the relevant point on the timeline
+  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
+  /// 1.0, so negative values and values greater than 1.0 are valid (and can
+  /// easily be generated by curves such as [Curves.elasticInOut]). Each channel
+  /// will be clamped to the range 0 to 255.
+  ///
+  /// Values for `t` are usually obtained from an [Animation<double>], such as
+  /// an [AnimationController].
   static Color lerp(Color a, Color b, double t) {
+    assert(t != null);
     if (a == null && b == null)
       return null;
     if (a == null)
diff --git a/lib/ui/text.dart b/lib/ui/text.dart
index 8cd6a447ec604..2194c51bdd72b 100644
--- a/lib/ui/text.dart
+++ b/lib/ui/text.dart
@@ -62,8 +62,25 @@ class FontWeight {
   ///
   /// Rather than using fractional weights, the interpolation rounds to the
   /// nearest weight.
-  static FontWeight lerp(FontWeight begin, FontWeight end, double t) {
-    return values[lerpDouble(begin?.index ?? normal.index, end?.index ?? normal.index, t.clamp(0.0, 1.0)).round()];
+  ///
+  /// Any null values for `a` or `b` are interpreted as equivalent to [normal]
+  /// (also known as [w400]).
+  ///
+  /// The `t` argument represents position on the timeline, with 0.0 meaning
+  /// that the interpolation has not started, returning `a` (or something
+  /// equivalent to `a`), 1.0 meaning that the interpolation has finished,
+  /// returning `b` (or something equivalent to `b`), and values in between
+  /// meaning that the interpolation is at the relevant point on the timeline
+  /// between `a` and `b`. The interpolation can be extrapolated beyond 0.0 and
+  /// 1.0, so negative values and values greater than 1.0 are valid (and can
+  /// easily be generated by curves such as [Curves.elasticInOut]). The result
+  /// is clamped to the range [w100]–[w900].
+  ///
+  /// Values for `t` are usually obtained from an [Animation<double>], such as
+  /// an [AnimationController].
+  static FontWeight lerp(FontWeight a, FontWeight b, double t) {
+    assert(t != null);
+    return values[lerpDouble(a?.index ?? normal.index, b?.index ?? normal.index, t).round().clamp(0, 8)];
   }
 
   String toString() {