diff --git a/README.md b/README.md
index 0c2386207d..5395202b4e 100644
--- a/README.md
+++ b/README.md
@@ -1185,7 +1185,7 @@ The box mark is a composite mark consisting of four marks:
 
 * a [rule](#rule) representing the extreme values (not including outliers)
 * a [bar](#bar) representing the interquartile range (trimmed to the data)
-* a [tick](#tick) represent the median value, and
+* a [tick](#tick) representing the median value, and
 * a [dot](#dot) representing outliers, if any
 
 The given *options* are passed through to these underlying marks, with the exception of the following options:
diff --git a/src/marks/box.d.ts b/src/marks/box.d.ts
index 1a55e72309..f579b3f9a0 100644
--- a/src/marks/box.d.ts
+++ b/src/marks/box.d.ts
@@ -4,10 +4,50 @@ import type {DotOptions} from "./dot.js";
 import type {RuleXOptions, RuleYOptions} from "./rule.js";
 import type {TickXOptions, TickYOptions} from "./tick.js";
 
+/** Options for the boxX mark. */
 export type BoxXOptions = DotOptions & BarXOptions & TickXOptions & RuleXOptions;
 
+/** Options for the boxY mark. */
 export type BoxYOptions = DotOptions & BarYOptions & TickYOptions & RuleYOptions;
 
+/**
+ * Returns a box mark that draws horizontal boxplots where **x** is quantitative
+ * or temporal and **y**, if present, is ordinal. The box mark is a compound
+ * mark consisting of four marks:
+ *
+ * - a rule representing the extreme values (not including outliers),
+ * - a bar representing the interquartile range (trimmed to the data),
+ * - a tick representing the median value, and
+ * - a dot representing outliers, if any.
+ *
+ * The given *options* are passed through to these underlying marks, with the
+ * exception of the following options:
+ *
+ * - **fill** - the fill color of the bar; defaults to gray
+ * - **fillOpacity** - the fill opacity of the bar; defaults to 1
+ * - **stroke** - the stroke color of the rule, tick, and dot; defaults to *currentColor*
+ * - **strokeOpacity** - the stroke opacity of the rule, tick, and dot; defaults to 1
+ * - **strokeWidth** - the stroke width of the tick; defaults to 2
+ */
 export function boxX(data?: Data, options?: BoxXOptions): CompoundMark;
 
+/**
+ * Returns a box mark that draws vertical boxplots where **y** is quantitative
+ * or temporal and **x**, if present, is ordinal. The box mark is a compound
+ * mark consisting of four marks:
+ *
+ * - a rule representing the extreme values (not including outliers),
+ * - a bar representing the interquartile range (trimmed to the data),
+ * - a tick representing the median value, and
+ * - a dot representing outliers, if any.
+ *
+ * The given *options* are passed through to these underlying marks, with the
+ * exception of the following options:
+ *
+ * - **fill** - the fill color of the bar; defaults to gray
+ * - **fillOpacity** - the fill opacity of the bar; defaults to 1
+ * - **stroke** - the stroke color of the rule, tick, and dot; defaults to *currentColor*
+ * - **strokeOpacity** - the stroke opacity of the rule, tick, and dot; defaults to 1
+ * - **strokeWidth** - the stroke width of the tick; defaults to 2
+ */
 export function boxY(data?: Data, options?: BoxYOptions): CompoundMark;
diff --git a/src/marks/box.js b/src/marks/box.js
index 230f0ee948..be73af0e8d 100644
--- a/src/marks/box.js
+++ b/src/marks/box.js
@@ -1,5 +1,6 @@
-import {min, max, quantile} from "d3";
+import {max, min, quantile} from "d3";
 import {marks} from "../mark.js";
+import {identity} from "../options.js";
 import {groupX, groupY, groupZ} from "../transforms/group.js";
 import {map} from "../transforms/map.js";
 import {barX, barY} from "./bar.js";
@@ -11,7 +12,7 @@ export function boxX(data, options = {}) {
   // Returns a composite mark for producing a horizontal box plot, applying the
   // necessary statistical transforms. The boxes are grouped by y, if present.
   const {
-    x = {transform: (x) => x},
+    x = identity,
     y = null,
     fill = "#ccc",
     fillOpacity,
@@ -34,7 +35,7 @@ export function boxY(data, options = {}) {
   // Returns a composite mark for producing a vertical box plot, applying the
   // necessary statistical transforms. The boxes are grouped by x, if present.
   const {
-    y = {transform: (y) => y},
+    y = identity,
     x = null,
     fill = "#ccc",
     fillOpacity,
@@ -60,20 +61,20 @@ function oqr(values) {
   return values.map((v) => (v < r1 || v > r2 ? v : NaN));
 }
 
-function loqr1(values, value) {
-  const lo = quartile1(values, value) * 2.5 - quartile3(values, value) * 1.5;
+function loqr1(values) {
+  const lo = quartile1(values) * 2.5 - quartile3(values) * 1.5;
   return min(values, (d) => (d >= lo ? d : NaN));
 }
 
-function hiqr2(values, value) {
-  const hi = quartile3(values, value) * 2.5 - quartile1(values, value) * 1.5;
+function hiqr2(values) {
+  const hi = quartile3(values) * 2.5 - quartile1(values) * 1.5;
   return max(values, (d) => (d <= hi ? d : NaN));
 }
 
-function quartile1(values, value) {
-  return quantile(values, 0.25, value);
+function quartile1(values) {
+  return quantile(values, 0.25);
 }
 
-function quartile3(values, value) {
-  return quantile(values, 0.75, value);
+function quartile3(values) {
+  return quantile(values, 0.75);
 }