diff --git a/MultiversalDiplomacy/Model/Season.cs b/MultiversalDiplomacy/Model/Season.cs
new file mode 100644
index 0000000..50721bc
--- /dev/null
+++ b/MultiversalDiplomacy/Model/Season.cs
@@ -0,0 +1,109 @@
+namespace MultiversalDiplomacy.Model;
+
+/// <summary>
+/// Represents a state of the map produced by a set of move orders on a previous season.
+/// </summary>
+public class Season
+{
+    /// <summary>
+    /// A shared counter for handing out new timeline numbers.
+    /// </summary>
+    private class TimelineFactory
+    {
+        private int nextTimeline = 0;
+
+        public int NextTimeline() => nextTimeline++;
+    }
+
+    /// <summary>
+    /// The first turn number.
+    /// </summary>
+    public const int FIRST_TURN = 0;
+
+    /// <summary>
+    /// The season immediately preceding this season.
+    /// If this season is an alternate timeline root, the past is from the origin timeline.
+    /// The initial season does not have a past.
+    /// </summary>
+    public Season? Past { get; }
+
+    /// <summary>
+    /// The current turn, beginning at 0. Each season (spring and fall) is one turn.
+    /// Phases that only occur after the fall phase occur when Turn % 2 == 1.
+    /// The current year is (Turn / 2) + 1901.
+    /// </summary>
+    public int Turn { get; }
+
+    /// <summary>
+    /// The timeline to which this season belongs.
+    /// </summary>
+    public int Timeline { get; }
+
+    /// <summary>
+    /// The shared timeline number generator.
+    /// </summary>
+    private TimelineFactory Timelines { get; }
+
+    private Season(Season? past, int turn, int timeline, TimelineFactory factory)
+    {
+        this.Past = past;
+        this.Turn = turn;
+        this.Timeline = timeline;
+        this.Timelines = factory;
+    }
+
+    /// <summary>
+    /// Create a root season at the beginning of time.
+    /// </summary>
+    public static Season MakeRoot()
+    {
+        TimelineFactory factory = new TimelineFactory();
+        return new Season(
+            past: null,
+            turn: FIRST_TURN,
+            timeline: factory.NextTimeline(),
+            factory: factory);
+    }
+
+    /// <summary>
+    /// Create a season immediately after this one in the same timeline.
+    /// </summary>
+    public Season MakeNext()
+        => new Season(this, this.Turn + 1, this.Timeline, this.Timelines);
+
+    /// <summary>
+    /// Create a season immediately after this one in a new timeline.
+    /// </summary>
+    public Season MakeFork()
+        => new Season(this, this.Turn + 1, this.Timelines.NextTimeline(), this.Timelines);
+
+    /// <summary>
+    /// Returns the first season in this season's timeline. The first season is the
+    /// root of the first timeline. The earliest season in each alternate timeline is
+    /// the root of that timeline.
+    /// </summary>
+    public Season TimelineRoot()
+        => this.Past != null && this.Timeline == this.Past.Timeline
+            ? this.Past.TimelineRoot()
+            : this;
+
+    /// <summary>
+    /// Returns whether this season is in an adjacent timeline to another season.
+    /// </summary>
+    public bool InAdjacentTimeline(Season other)
+    {
+        // Timelines are adjacent to themselves. Early out in that case.
+        if (this.Timeline == other.Timeline) return true;
+
+        // If the timelines aren't identical, one of them isn't the initial trunk.
+        // They can still be adjacent if one of them branched off of the other, or
+        // if they both branched off of the same point.
+        Season thisRoot = this.TimelineRoot();
+        Season otherRoot = other.TimelineRoot();
+        return // One branched off the other
+                thisRoot.Past?.Timeline == other.Timeline
+            || otherRoot.Past?.Timeline == this.Timeline
+                // Both branched off of the same point
+            || thisRoot.Past == otherRoot.Past;
+    }
+}
diff --git a/MultiversalDiplomacyTests/SeasonTests.cs b/MultiversalDiplomacyTests/SeasonTests.cs
new file mode 100644
index 0000000..0bb2556
--- /dev/null
+++ b/MultiversalDiplomacyTests/SeasonTests.cs
@@ -0,0 +1,50 @@
+using MultiversalDiplomacy.Model;
+
+using NUnit.Framework;
+
+namespace MultiversalDiplomacyTests;
+
+public class SeasonTests
+{
+    [Test]
+    public void TimelineForking()
+    {
+        Season a0 = Season.MakeRoot();
+        Season a1 = a0.MakeNext();
+        Season a2 = a1.MakeNext();
+        Season a3 = a2.MakeNext();
+        Season b1 = a1.MakeFork();
+        Season b2 = b1.MakeNext();
+        Season c1 = a1.MakeFork();
+        Season d1 = a2.MakeFork();
+
+        Assert.That(a0.Timeline, Is.EqualTo(0), "Unexpected trunk timeline number");
+        Assert.That(a1.Timeline, Is.EqualTo(0), "Unexpected trunk timeline number");
+        Assert.That(a2.Timeline, Is.EqualTo(0), "Unexpected trunk timeline number");
+        Assert.That(a3.Timeline, Is.EqualTo(0), "Unexpected trunk timeline number");
+        Assert.That(b1.Timeline, Is.EqualTo(1), "Unexpected first alt number");
+        Assert.That(b2.Timeline, Is.EqualTo(1), "Unexpected first alt number");
+        Assert.That(c1.Timeline, Is.EqualTo(2), "Unexpected second alt number");
+        Assert.That(d1.Timeline, Is.EqualTo(3), "Unexpected third alt number");
+
+        Assert.That(a0.Turn, Is.EqualTo(Season.FIRST_TURN + 0), "Unexpected first turn number");
+        Assert.That(a1.Turn, Is.EqualTo(Season.FIRST_TURN + 1), "Unexpected next turn number");
+        Assert.That(a2.Turn, Is.EqualTo(Season.FIRST_TURN + 2), "Unexpected next turn number");
+        Assert.That(a3.Turn, Is.EqualTo(Season.FIRST_TURN + 3), "Unexpected next turn number");
+        Assert.That(b1.Turn, Is.EqualTo(Season.FIRST_TURN + 2), "Unexpected fork turn number");
+        Assert.That(b2.Turn, Is.EqualTo(Season.FIRST_TURN + 3), "Unexpected fork turn number");
+        Assert.That(c1.Turn, Is.EqualTo(Season.FIRST_TURN + 2), "Unexpected fork turn number");
+        Assert.That(d1.Turn, Is.EqualTo(Season.FIRST_TURN + 3), "Unexpected fork turn number");
+
+        Assert.That(a0.TimelineRoot(), Is.EqualTo(a0), "Expected timeline root to be reflexive");
+        Assert.That(a3.TimelineRoot(), Is.EqualTo(a0), "Expected trunk timeline to have root");
+        Assert.That(b1.TimelineRoot(), Is.EqualTo(b1), "Expected alt timeline root to be reflexive");
+        Assert.That(b2.TimelineRoot(), Is.EqualTo(b1), "Expected alt timeline to root at first fork");
+        Assert.That(c1.TimelineRoot(), Is.EqualTo(c1), "Expected alt timeline root to be reflexive");
+        Assert.That(d1.TimelineRoot(), Is.EqualTo(d1), "Expected alt timeline root to be reflexive");
+
+        Assert.That(b2.InAdjacentTimeline(a3), Is.True, "Expected alts to be adjacent to origin");
+        Assert.That(b2.InAdjacentTimeline(c1), Is.True, "Expected alts with common origin to be adjacent");
+        Assert.That(b2.InAdjacentTimeline(d1), Is.False, "Expected alts from different origins not to be adjacent");
+    }
+}
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