diff --git a/go/delivery/window.go b/go/delivery/window.go
new file mode 100644
index 0000000..d5c7a00
--- /dev/null
+++ b/go/delivery/window.go
@@ -0,0 +1,64 @@
+package delivery
+
+import (
+	"slices"
+	"sort"
+	"time"
+)
+
+// WindowHeld is the part of a window every window keeper shares (design 51: distribution's old seat name and direct
+// writes, the builder's old reference form): whether it is open, the nodes holding it and since when, and when it last
+// closed and who held it then. A keeper embeds it in its own record, so its JSON is the record's.
+type WindowHeld struct {
+	Open    bool                 `json:"open"`
+	Holding []string             `json:"holding,omitempty"`
+	Since   map[string]time.Time `json:"since,omitempty"`
+	Closed  time.Time            `json:"closed,omitzero"`
+	// LastHolding are the nodes that held it last, said when it closes.
+	LastHolding []string `json:"last_holding,omitempty"`
+}
+
+// StepWindow is the one rule every window keeper steps its window by (security-upgrade review F3, the second of a
+// kind): named is every node the read named, dropped ones included, and holding those of them that hold the window at
+// now. A read that names no node judges nothing, as a first view's empty answer at revision 0 is no empty state (ADR
+// 0615): an empty node_haves or a placement not yet read never closes a window, and the window stays as it was. A read
+// naming nodes, none holding, closes an open window (closed true, once); one naming a holder opens it, each holder's
+// since kept from w. overdue are the holders whose since is at least findingAfter before now (the keeper's finding,
+// cleared for every other node); a read naming no node keeps w's holders' overdue as they stand at now.
+func StepWindow(w WindowHeld, named, holding []string, now time.Time, findingAfter time.Duration) (next WindowHeld, closed bool, overdue []string) {
+	now = now.UTC()
+	if len(named) == 0 {
+		next = w
+		for _, n := range w.Holding {
+			if since, ok := w.Since[n]; ok && now.Sub(since) >= findingAfter {
+				overdue = append(overdue, n)
+			}
+		}
+		return next, false, overdue
+	}
+	if len(holding) == 0 {
+		next = WindowHeld{Closed: w.Closed, LastHolding: w.LastHolding}
+		if w.Open {
+			next.Closed, next.LastHolding = now, slices.Clone(w.Holding)
+			if next.LastHolding == nil {
+				next.LastHolding = []string{}
+			}
+			return next, true, nil
+		}
+		return next, false, nil
+	}
+	held := slices.Clone(holding)
+	sort.Strings(held)
+	next = WindowHeld{Open: true, Holding: held, Since: map[string]time.Time{}, Closed: w.Closed, LastHolding: w.LastHolding}
+	for _, n := range held {
+		since, had := w.Since[n]
+		if !had {
+			since = now
+		}
+		next.Since[n] = since
+		if now.Sub(since) >= findingAfter {
+			overdue = append(overdue, n)
+		}
+	}
+	return next, false, overdue
+}
diff --git a/go/delivery/window_test.go b/go/delivery/window_test.go
new file mode 100644
index 0000000..8fd1ade
--- /dev/null
+++ b/go/delivery/window_test.go
@@ -0,0 +1,41 @@
+package delivery
+
+import (
+	"slices"
+	"testing"
+	"time"
+)
+
+// The one rule every window keeper steps by (design 51; security-upgrade review F3): a read naming no node judges
+// nothing, so an empty node_haves or an unread placement never closes a window.
+func TestStepWindow(t *testing.T) {
+	now := time.Date(2026, 11, 4, 17, 0, 0, 0, time.UTC)
+	week := 7 * 24 * time.Hour
+	long := now.Add(-8 * 24 * time.Hour)
+	open := WindowHeld{Open: true, Holding: []string{"n1"}, Since: map[string]time.Time{"n1": long}}
+	for _, c := range []struct {
+		name           string
+		w              WindowHeld
+		named, holding []string
+		open, closed   bool
+		holds, overdue []string
+	}{
+		{"a read naming no node leaves an open window open, its holder still overdue", open, nil, nil, true, false, []string{"n1"}, []string{"n1"}},
+		{"a read naming no node leaves a closed window closed", WindowHeld{Closed: long}, nil, nil, false, false, nil, nil},
+		{"a read naming nodes, none holding, closes an open window once", open, []string{"n1", "n2"}, nil, false, true, nil, nil},
+		{"a closed window a read finds unheld stays closed, said no more", WindowHeld{Closed: long, LastHolding: []string{"n1"}}, []string{"n1"}, nil, false, false, nil, nil},
+		{"a new holder opens it, since now, not overdue", WindowHeld{}, []string{"n1", "n2"}, []string{"n2"}, true, false, []string{"n2"}, nil},
+		{"a holder keeps its since and is overdue past the bound", open, []string{"n1", "n2"}, []string{"n2", "n1"}, true, false, []string{"n1", "n2"}, []string{"n1"}},
+	} {
+		next, closed, overdue := StepWindow(c.w, c.named, c.holding, now, week)
+		if next.Open != c.open || closed != c.closed || !slices.Equal(next.Holding, c.holds) || !slices.Equal(overdue, c.overdue) {
+			t.Errorf("%s: open %v closed %v holding %v overdue %v", c.name, next.Open, closed, next.Holding, overdue)
+		}
+		if closed && (!next.Closed.Equal(now) || !slices.Equal(next.LastHolding, c.w.Holding)) {
+			t.Errorf("%s: closed at %v, last held by %v", c.name, next.Closed, next.LastHolding)
+		}
+		if !c.open && !closed && !next.Closed.Equal(c.w.Closed) {
+			t.Errorf("%s: its last close moved to %v", c.name, next.Closed)
+		}
+	}
+}