diff --git a/index.ts b/index.ts
index e3a4d39..9477ab2 100644
--- a/index.ts
+++ b/index.ts
@@ -1,2 +1,3 @@
+export * as r1 from './r1/_index'
 export * as r2 from './r2/_index'
 export * as s2 from './s2/_index'
diff --git a/r1/Interval.ts b/r1/Interval.ts
new file mode 100644
index 0000000..166cc77
--- /dev/null
+++ b/r1/Interval.ts
@@ -0,0 +1,172 @@
+/**
+ * Interval represents a closed interval on ℝ.
+ * Zero-length intervals (where lo == hi) represent single points.
+ * If lo > hi then the interval is empty.
+ */
+export class Interval {
+  lo: number = 0.0
+  hi: number = 0.0
+
+  /**
+   * Returns a new Interval.
+   * @category Constructors
+   */
+  constructor(lo: number, hi: number) {
+    this.lo = lo
+    this.hi = hi
+  }
+
+  /**
+   * Reports whether the interval is empty.
+   */
+  isEmpty(): boolean {
+    return this.lo > this.hi
+  }
+
+  /**
+   * Returns true iff the interval contains the same points as oi.
+   */
+  equal(oi: Interval): boolean {
+    return (this.lo == oi.lo && this.hi == oi.hi) || (this.isEmpty() && oi.isEmpty())
+  }
+
+  /**
+   * Returns the midpoint of the interval.
+   * Behaviour is undefined for empty intervals.
+   */
+  center(): number {
+    return 0.5 * (this.lo + this.hi)
+  }
+
+  /**
+   * Returns the length of the interval.
+   * The length of an empty interval is negative.
+   */
+  length(): number {
+    return this.hi - this.lo
+  }
+
+  /**
+   * Returns true iff the interval contains p.
+   */
+  contains(p: number): boolean {
+    return this.lo <= p && p <= this.hi
+  }
+
+  /**
+   * Returns true iff the interval contains oi.
+   */
+  containsInterval(oi: Interval): boolean {
+    if (oi.isEmpty()) return true
+    return this.lo <= oi.lo && oi.hi <= this.hi
+  }
+
+  /**
+   * Returns true iff the interval strictly contains p.
+   */
+  interiorContains(p: number): boolean {
+    return this.lo < p && p < this.hi
+  }
+
+  /**
+   * Returns true iff the interval strictly contains oi.
+   */
+  interiorContainsInterval(oi: Interval): boolean {
+    if (oi.isEmpty()) return true
+    return this.lo < oi.lo && oi.hi < this.hi
+  }
+
+  /**
+   * Returns true iff the interval contains any points in common with oi.
+   */
+  intersects(oi: Interval): boolean {
+    if (this.lo <= oi.lo) return oi.lo <= this.hi && oi.lo <= oi.hi // oi.lo ∈ i and oi is not empty
+    return this.lo <= oi.hi && this.lo <= this.hi // i.lo ∈ oi and i is not empty
+  }
+
+  /**
+   * Returns true iff the interior of the interval contains any points in common with oi, including the latter's boundary.
+   */
+  interiorIntersects(oi: Interval): boolean {
+    return oi.lo < this.hi && this.lo < oi.hi && this.lo < this.hi && oi.lo <= oi.hi
+  }
+
+  /**
+   * Returns the interval containing all points common to i and j.
+   * @note Empty intervals do not need to be special-cased.
+   */
+  intersection(j: Interval): Interval {
+    return new Interval(Math.max(this.lo, j.lo), Math.min(this.hi, j.hi))
+  }
+
+  /**
+   * Returns the smallest interval that contains this interval and the given interval.
+   */
+  union(other: Interval): Interval {
+    if (this.isEmpty()) return other
+    if (other.isEmpty()) return this
+    return new Interval(Math.min(this.lo, other.lo), Math.max(this.hi, other.hi))
+  }
+
+  /**
+   * Returns the interval expanded so that it contains the given point.
+   */
+  addPoint(p: number): Interval {
+    if (this.isEmpty()) return new Interval(p, p)
+    if (p < this.lo) return new Interval(p, this.hi)
+    if (p > this.hi) return new Interval(this.lo, p)
+    return this
+  }
+
+  /**
+   * Returns the closest point in the interval to the given point p.
+   * The interval must be non-empty.
+   */
+  clampPoint(p: number): number {
+    return Math.max(this.lo, Math.min(this.hi, p))
+  }
+
+  /**
+   * Returns an interval that has been expanded on each side by margin.
+   * If margin is negative, then the function shrinks the interval on each side by margin instead.
+   * The resulting interval may be empty.
+   * Any expansion of an empty interval remains empty.
+   */
+  expanded(margin: number): Interval {
+    if (this.isEmpty()) return this
+    return new Interval(this.lo - margin, this.hi + margin)
+  }
+
+  /**
+   * Truncates {lo, hi} floats to n digits of precision.
+   */
+  trunc(n: number = 15): Interval {
+    const p = Number(`1e${n}`)
+    const trunc = (dim: number) => Math.round(dim * p) / p
+    return new Interval(trunc(this.lo), trunc(this.hi))
+  }
+
+  /**
+   * Generates a human readable string.
+   */
+  toString(): string {
+    const t = this.trunc(7)
+    return `[${t.lo.toFixed(7)}, ${t.hi.toFixed(7)}]`
+  }
+
+  /**
+   * Returns an empty interval.
+   * @category Constructors
+   */
+  static empty(): Interval {
+    return new Interval(1, 0)
+  }
+
+  /**
+   * Returns an interval representing a single point.
+   * @category Constructors
+   */
+  static fromPoint(p: number): Interval {
+    return new Interval(p, p)
+  }
+}
diff --git a/r1/Interval_test.ts b/r1/Interval_test.ts
new file mode 100644
index 0000000..1204126
--- /dev/null
+++ b/r1/Interval_test.ts
@@ -0,0 +1,125 @@
+import test from 'node:test'
+import { ok, equal } from 'node:assert/strict'
+import { Interval } from './Interval'
+
+// Some standard intervals for use throughout the tests.
+const UNIT = new Interval(0, 1)
+const NEG_UNIT = new Interval(-1, 0)
+const HALF = new Interval(0.5, 0.5)
+const EMPTY = Interval.empty()
+
+test('isEmpty', t => {
+  ok(!UNIT.isEmpty(), 'should not be empty')
+  ok(!NEG_UNIT.isEmpty(), 'should not be empty')
+  ok(!HALF.isEmpty(), 'should not be empty')
+  ok(EMPTY.isEmpty(), 'should not empty')
+})
+
+test('center', t => {
+  equal(UNIT.center(), 0.5)
+  equal(NEG_UNIT.center(), -0.5)
+  equal(HALF.center(), 0.5)
+})
+
+test('length', t => {
+  equal(UNIT.length(), 1)
+  equal(NEG_UNIT.length(), 1)
+  equal(HALF.length(), 0)
+})
+
+test('contains', t => {
+  ok(UNIT.contains(0.5))
+  ok(UNIT.interiorContains(0.5))
+
+  ok(UNIT.contains(0))
+  ok(!UNIT.interiorContains(0))
+
+  ok(UNIT.contains(1))
+  ok(!UNIT.interiorContains(1))
+})
+
+test('operations', t => {
+  ok(EMPTY.containsInterval(EMPTY))
+  ok(EMPTY.interiorContainsInterval(EMPTY))
+  ok(!EMPTY.intersects(EMPTY))
+  ok(!EMPTY.interiorIntersects(EMPTY))
+
+  ok(!EMPTY.containsInterval(UNIT))
+  ok(!EMPTY.interiorContainsInterval(UNIT))
+  ok(!EMPTY.intersects(UNIT))
+  ok(!EMPTY.interiorIntersects(UNIT))
+
+  ok(UNIT.containsInterval(HALF))
+  ok(UNIT.interiorContainsInterval(HALF))
+  ok(UNIT.intersects(HALF))
+  ok(UNIT.interiorIntersects(HALF))
+
+  ok(UNIT.containsInterval(UNIT))
+  ok(!UNIT.interiorContainsInterval(UNIT))
+  ok(UNIT.intersects(UNIT))
+  ok(UNIT.interiorIntersects(UNIT))
+
+  ok(UNIT.containsInterval(EMPTY))
+  ok(UNIT.interiorContainsInterval(EMPTY))
+  ok(!UNIT.intersects(EMPTY))
+  ok(!UNIT.interiorIntersects(EMPTY))
+
+  ok(!UNIT.containsInterval(NEG_UNIT))
+  ok(!UNIT.interiorContainsInterval(NEG_UNIT))
+  ok(UNIT.intersects(NEG_UNIT))
+  ok(!UNIT.interiorIntersects(NEG_UNIT))
+
+  const i = new Interval(0, 0.5)
+  ok(UNIT.containsInterval(i))
+  ok(!UNIT.interiorContainsInterval(i))
+  ok(UNIT.intersects(i))
+  ok(UNIT.interiorIntersects(i))
+
+  ok(!HALF.containsInterval(i))
+  ok(!HALF.interiorContainsInterval(i))
+  ok(HALF.intersects(i))
+  ok(!HALF.interiorIntersects(i))
+})
+
+test('intersection', t => {
+  ok(UNIT.intersection(HALF).equal(HALF))
+  ok(UNIT.intersection(NEG_UNIT).equal(new Interval(0, 0)))
+  ok(NEG_UNIT.intersection(HALF).equal(EMPTY))
+  ok(UNIT.intersection(EMPTY).equal(EMPTY))
+  ok(EMPTY.intersection(UNIT).equal(EMPTY))
+})
+
+test('union', t => {
+  ok(new Interval(99, 100).union(EMPTY).equal(new Interval(99, 100)))
+  ok(EMPTY.union(new Interval(99, 100)).equal(new Interval(99, 100)))
+  ok(new Interval(5, 3).union(new Interval(0, -2)).equal(EMPTY))
+  ok(new Interval(0, -2).union(new Interval(5, 3)).equal(EMPTY))
+  ok(UNIT.union(UNIT).equal(UNIT))
+  ok(UNIT.union(NEG_UNIT).equal(new Interval(-1, 1)))
+  ok(NEG_UNIT.union(UNIT).equal(new Interval(-1, 1)))
+  ok(HALF.union(UNIT).equal(UNIT))
+})
+
+test('addPoint', t => {
+  ok(EMPTY.addPoint(5).equal(new Interval(5, 5)))
+  ok(new Interval(5, 5).addPoint(-1).equal(new Interval(-1, 5)))
+  ok(new Interval(-1, 5).addPoint(0).equal(new Interval(-1, 5)))
+  ok(new Interval(-1, 5).addPoint(6).equal(new Interval(-1, 6)))
+})
+
+test('clampPoint', t => {
+  equal(new Interval(0.1, 0.4).clampPoint(0.3), 0.3)
+  equal(new Interval(0.1, 0.4).clampPoint(-7.0), 0.1)
+  equal(new Interval(0.1, 0.4).clampPoint(0.6), 0.4)
+})
+
+test('expanded', t => {
+  ok(EMPTY.expanded(0.45).equal(EMPTY))
+  ok(UNIT.expanded(0.5).equal(new Interval(-0.5, 1.5)))
+  ok(UNIT.expanded(-0.5).equal(new Interval(0.5, 0.5)))
+  ok(UNIT.expanded(-0.51).equal(EMPTY))
+})
+
+test('toString', t => {
+  equal(new Interval(2, 4.5).toString(), '[2.0000000, 4.5000000]')
+})
diff --git a/r1/_index.ts b/r1/_index.ts
new file mode 100644
index 0000000..6430173
--- /dev/null
+++ b/r1/_index.ts
@@ -0,0 +1,5 @@
+/**
+ * Module r1 implements types and functions for working with geometry in ℝ¹.
+ * @module r1
+ */
+export { Interval } from './Interval'