diff --git a/build.zig.zon b/build.zig.zon
index fb7ffa8..8aa8c1d 100644
--- a/build.zig.zon
+++ b/build.zig.zon
@@ -9,12 +9,12 @@
             .hash = "webidl_zig-0.1.0-FrIyV_uUBgAGtNwP-OkLegPJMWUnpeKuPqDdVbWyC9Fn",
         },
         .lattice = .{
-            .url = "git+https://github.com/Midstall/lattice#17053e425b5e472f3a6a57a382fad18b0b4ea356",
-            .hash = "lattice-0.1.0-TdtxHEviCQCHdTwR8eVLuDYwIIKcxFQLjQs18onvIC1q",
+            .url = "git+https://github.com/Midstall/lattice#e22083d29efc14d30887888eded76424bf25b72e",
+            .hash = "lattice-0.1.0-TdtxHEviCQCZaOH7y3ls5kW8sWTefXfiT8g-MUJy913z",
         },
         .prism = .{
-            .url = "git+https://github.com/Midstall/prism#f24fce92e8306e581c1e6fa6fe36c924183bab6a",
-            .hash = "prism-0.1.0-PSE12AbIOAAMnhpTrIYz4xOe1wphrH0hdEBig6v50waz",
+            .url = "git+https://github.com/Midstall/prism#2b50ec14c7e6c0795c22f9778483d0954ff7550f",
+            .hash = "prism-0.1.0-PSE12EzaOADdJQZq2Z9qoEC978am8KnR-bX117W6SpoQ",
         },
     },
     .paths = .{
diff --git a/flake.nix b/flake.nix
index 731d514..a5a41e3 100644
--- a/flake.nix
+++ b/flake.nix
@@ -80,7 +80,7 @@
 
             zigDeps = pkgs.zig.fetchDeps {
               inherit (finalAttrs) src pname version;
-              hash = "sha256-yj/4cpEwPMhbKvgnichxCyAILE/FE9wnb0VEuK1x/og=";
+              hash = "sha256-Xt8LfYQ9tks5hb2x9GsM5a1kMQFKUzX7kKC2BNUTbd4=";
             };
 
             nativeBuildInputs = with pkgs; [
diff --git a/lib/phantom/backend/tui_cells.zig b/lib/phantom/backend/tui_cells.zig
index 0816b53..e9d6ce6 100644
--- a/lib/phantom/backend/tui_cells.zig
+++ b/lib/phantom/backend/tui_cells.zig
@@ -11,6 +11,7 @@ const DisplayList = dl.DisplayList;
 const text = @import("../text.zig");
 const mono = @import("../text/mono.zig");
 const image_mod = @import("../image/Image.zig");
+const icon_builtin = @import("../icon/builtin.zig");
 const Attrs = grid_mod.Attrs;
 
 pub const Ctx = struct {
@@ -274,7 +275,6 @@ fn averageColor(img: *image_mod) Rgb {
 }
 
 fn paintIconFallback(grid: *CellGrid, p: dl.IconPrimitive, ctx: Ctx) void {
-    // One solid block. The icon shape needs a graphics protocol, and mode B has none.
     const rect = geom.PhysicalRect{
         .x = p.origin.x,
         .y = p.origin.y,
@@ -283,9 +283,42 @@ fn paintIconFallback(grid: *CellGrid, p: dl.IconPrimitive, ctx: Ctx) void {
     };
     const b = cellBounds(rect, ctx, grid);
     const color = Rgb.fromColor(p.color);
+
+    // A mark that has no coverage draws nothing, which is what the block below
+    // does at alpha zero. Any coverage above that draws the whole character: a
+    // cell is either the character or it is not, so there is no partly drawn
+    // one. `paintText` treats its own colour the same way.
+    if (p.color.a <= 0) return;
+
+    if (icon_builtin.cellMarkFor(p.id)) |mark| {
+        paintCellMark(grid, b, mark, color);
+        return;
+    }
+    // No character means this mark, so it degrades to one solid block. The icon
+    // shape needs a graphics protocol, and mode B has none.
     fillCellRect(grid, b, color, p.color.a);
 }
 
+/// Put `mark` into the cells of `b`, one character for a symbol and the whole
+/// box for a rule.
+fn paintCellMark(grid: *CellGrid, b: Bounds, mark: icon_builtin.CellMark, color: Rgb) void {
+    // An empty box means the mark sits off the grid, or rounded away to nothing.
+    if (b.c1 <= b.c0 or b.r1 <= b.r0) return;
+    if (mark.tile) {
+        var row = b.r0;
+        while (row < b.r1) : (row += 1) {
+            var col = b.c0;
+            while (col < b.c1) : (col += 1) grid.putChar(col, row, mark.cp, color, .{});
+        }
+        return;
+    }
+    // The middle of the box, biased left and up. A square mark is two cells wide
+    // whenever a cell is taller than it is wide, which is every terminal, and
+    // the box then has no true middle. Left is the better of the two: a mark
+    // usually leads the text beside it, and the far cell would put a gap there.
+    grid.putChar(b.c0 + (b.c1 - b.c0 - 1) / 2, b.r0 + (b.r1 - b.r0 - 1) / 2, mark.cp, color, .{});
+}
+
 /// Shift a rect by the current scroll offset. `RenderScrollView.paintFn` paints its
 /// child at the plain, unscrolled offset and leaves the scroll to the backend (see
 /// `prism.zig`'s `appendQuad` doing the same for the GPU path), so every primitive
@@ -1015,3 +1048,113 @@ test "an extreme scroll offset does not crash the text or rect paths" {
     try render(&g, list, .{ .cell_w = 8, .cell_h = 16 });
     try std.testing.expectEqual(@as(u8, 0), g.cellAt(0, 0).?.bg.r);
 }
+
+test "a mark with a character becomes that character, not a block" {
+    const gpa = std.testing.allocator;
+    var g = try CellGrid.init(gpa, 8, 8);
+    defer g.deinit();
+    g.clear(.{ .r = 0, .g = 0, .b = 0 });
+
+    var list: DisplayList = .{};
+    defer list.deinit(gpa);
+    // One cell, at the origin.
+    try list.append(gpa, .{ .icon = .{
+        .id = .check,
+        .size = 16,
+        .color = .{ .r = 1, .g = 0, .b = 0, .a = 1 },
+        .origin = .{ .x = 0, .y = 0 },
+    } });
+
+    try render(&g, list, .{ .cell_w = 8, .cell_h = 16 });
+    const cell = g.cellAt(0, 0).?;
+    try std.testing.expectEqual(@as(u21, '\u{2713}'), cell.ch);
+    try std.testing.expectEqual(@as(u8, 255), cell.fg.r);
+    // A block would have painted the background instead, which is the bug this
+    // test is about: the tick has to be readable, so the cell keeps its own.
+    try std.testing.expectEqual(@as(u8, 0), cell.bg.r);
+}
+
+test "a rule fills every cell it covers, so a stacked rail has no gap" {
+    const gpa = std.testing.allocator;
+    var g = try CellGrid.init(gpa, 8, 8);
+    defer g.deinit();
+    g.clear(.{ .r = 0, .g = 0, .b = 0 });
+
+    var list: DisplayList = .{};
+    defer list.deinit(gpa);
+    // Three cells tall, which is the shape a provenance rail asks for.
+    try list.append(gpa, .{ .icon = .{
+        .id = .rule_vertical,
+        .size = 48,
+        .color = .{ .r = 1, .g = 1, .b = 1, .a = 1 },
+        .origin = .{ .x = 0, .y = 0 },
+    } });
+
+    try render(&g, list, .{ .cell_w = 8, .cell_h = 16 });
+    for (0..3) |row| {
+        try std.testing.expectEqual(@as(u21, '\u{2502}'), g.cellAt(0, @intCast(row)).?.ch);
+    }
+}
+
+test "a symbol taller than one cell draws once, not once for every cell" {
+    const gpa = std.testing.allocator;
+    var g = try CellGrid.init(gpa, 8, 8);
+    defer g.deinit();
+    g.clear(.{ .r = 0, .g = 0, .b = 0 });
+
+    var list: DisplayList = .{};
+    defer list.deinit(gpa);
+    try list.append(gpa, .{ .icon = .{
+        .id = .check,
+        .size = 48,
+        .color = .{ .r = 1, .g = 1, .b = 1, .a = 1 },
+        .origin = .{ .x = 0, .y = 0 },
+    } });
+
+    try render(&g, list, .{ .cell_w = 8, .cell_h = 16 });
+    var ticks: usize = 0;
+    for (0..8) |row| {
+        for (0..8) |col| {
+            if (g.cellAt(@intCast(col), @intCast(row)).?.ch == '\u{2713}') ticks += 1;
+        }
+    }
+    try std.testing.expectEqual(@as(usize, 1), ticks);
+}
+
+test "a mark with no character keeps the block it always had" {
+    const gpa = std.testing.allocator;
+    var g = try CellGrid.init(gpa, 8, 8);
+    defer g.deinit();
+    g.clear(.{ .r = 0, .g = 0, .b = 0 });
+
+    var list: DisplayList = .{};
+    defer list.deinit(gpa);
+    try list.append(gpa, .{ .icon = .{
+        .id = .torii,
+        .size = 16,
+        .color = .{ .r = 1, .g = 0, .b = 0, .a = 1 },
+        .origin = .{ .x = 0, .y = 0 },
+    } });
+
+    try render(&g, list, .{ .cell_w = 8, .cell_h = 16 });
+    try std.testing.expectEqual(@as(u8, 255), g.cellAt(0, 0).?.bg.r);
+}
+
+test "a mark with no coverage draws nothing" {
+    const gpa = std.testing.allocator;
+    var g = try CellGrid.init(gpa, 8, 8);
+    defer g.deinit();
+    g.clear(.{ .r = 0, .g = 0, .b = 0 });
+
+    var list: DisplayList = .{};
+    defer list.deinit(gpa);
+    try list.append(gpa, .{ .icon = .{
+        .id = .check,
+        .size = 16,
+        .color = .{ .r = 1, .g = 0, .b = 0, .a = 0 },
+        .origin = .{ .x = 0, .y = 0 },
+    } });
+
+    try render(&g, list, .{ .cell_w = 8, .cell_h = 16 });
+    try std.testing.expectEqual(@as(u21, ' '), g.cellAt(0, 0).?.ch);
+}
diff --git a/lib/phantom/icon.zig b/lib/phantom/icon.zig
index 8ef9eaa..a6889b1 100644
--- a/lib/phantom/icon.zig
+++ b/lib/phantom/icon.zig
@@ -4,6 +4,8 @@ pub const stroke = @import("icon/stroke.zig");
 pub const builtin = @import("icon/builtin.zig");
 pub const Id = builtin.Id;
 pub const pathFor = builtin.pathFor;
+pub const CellMark = builtin.CellMark;
+pub const cellMarkFor = builtin.cellMarkFor;
 pub const torii = builtin.torii;
 pub const Point = path.Point;
 pub const Verb = path.Verb;
diff --git a/lib/phantom/icon/builtin.zig b/lib/phantom/icon/builtin.zig
index 99ee5ce..dd513a8 100644
--- a/lib/phantom/icon/builtin.zig
+++ b/lib/phantom/icon/builtin.zig
@@ -42,6 +42,51 @@ pub const Id = enum(u32) {
     rule_horizontal = 11,
 };
 
+/// What a cell backend draws in place of the mark.
+pub const CellMark = struct {
+    /// The codepoint to put in the cell. Every one of them is a single column
+    /// wide, so a mark never pushes the text beside it out of place.
+    cp: u21,
+    /// True for a mark that continues past its own box. A rule fills every cell
+    /// it covers, because a line with a gap in it is a different line. A symbol
+    /// draws once, in the cell at the middle of the box, because a tick
+    /// repeated four times is four ticks.
+    tile: bool = false,
+};
+
+/// The character a cell backend draws for `id`, or null when no character means
+/// the mark and the backend must fall back to a block.
+///
+/// A cell backend cannot draw a path, but it does not need to: the terminal
+/// draws text with its own font, and a text font has these. This is the reverse
+/// of the reason the paths exist (see `Id`). The bundled display faces have no
+/// tick, so pixel mode draws one itself; the terminal font has one, so cell
+/// mode asks for it by number.
+///
+/// The four chevrons come out as the geometric triangles. A terminal font is
+/// much more likely to have those than any of the chevron ornaments, and they
+/// give the same direction, which is all the mark says.
+pub fn cellMarkFor(id: Id) ?CellMark {
+    return switch (id) {
+        // The logomark is a drawing of a gate. No character stands for it, so a
+        // cell backend keeps its block.
+        .torii => null,
+        .check => .{ .cp = '\u{2713}' },
+        .cross => .{ .cp = '\u{2717}' },
+        .chevron_left => .{ .cp = '\u{25C0}' },
+        .chevron_right => .{ .cp = '\u{25B6}' },
+        .chevron_up => .{ .cp = '\u{25B2}' },
+        .chevron_down => .{ .cp = '\u{25BC}' },
+        .arrow_right => .{ .cp = '\u{2192}' },
+        // Plus and minus are ASCII on purpose. Every font has them, and the
+        // typographic minus buys nothing at the size of one cell.
+        .plus => .{ .cp = '+' },
+        .minus => .{ .cp = '-' },
+        .rule_vertical => .{ .cp = '\u{2502}', .tile = true },
+        .rule_horizontal => .{ .cp = '\u{2500}', .tile = true },
+    };
+}
+
 /// The centreline of `id`.
 pub fn pathFor(id: Id) path.Path {
     return switch (id) {
@@ -195,6 +240,7 @@ pub const torii = path.Path{ .verbs = &.{
 const std = @import("std");
 const stroke = @import("stroke.zig");
 const raster = @import("../text/raster.zig");
+const mono = @import("../text/mono.zig");
 
 /// Coverage of the bitmap cell holding the grid point (x, y), or 0 when that
 /// cell lies outside the bitmap, which is the same thing as no coverage.
@@ -461,3 +507,29 @@ test "the check mark is a tick and not a V: its vertex sits left of centre" {
     try std.testing.expectEqual(b.min_y, vertex.y);
     try std.testing.expect(vertex.x < grid / 2);
 }
+
+test "every mark a terminal font can draw is one column wide" {
+    // A mark two columns wide would push the text beside it out of place, and
+    // the cell backend has no way to know that happened.
+    inline for (@typeInfo(Id).@"enum".fields) |f| {
+        const id: Id = @enumFromInt(f.value);
+        if (cellMarkFor(id)) |mark| {
+            try std.testing.expectEqual(@as(u2, 1), mono.wcwidth(mark.cp));
+        }
+    }
+}
+
+test "only the rules tile, and every interface mark has a character" {
+    inline for (@typeInfo(Id).@"enum".fields) |f| {
+        const id: Id = @enumFromInt(f.value);
+        const mark = cellMarkFor(id);
+        if (id == .torii) {
+            // The logomark is a drawing, and no character means it.
+            try std.testing.expect(mark == null);
+            continue;
+        }
+        try std.testing.expect(mark != null);
+        const tiles = (id == .rule_vertical or id == .rule_horizontal);
+        try std.testing.expectEqual(tiles, mark.?.tile);
+    }
+}