diff --git a/README.md b/README.md
index a7f8be8..4de496c 100644
--- a/README.md
+++ b/README.md
@@ -10,9 +10,10 @@ A C++17 / Windows desktop application providing a **Maya Render Layers-style** U
 - **Scene Hierarchy** — Browse prim tree with per-type SVG icons; create, delete, rename prims; add/replace references
 - **Property Editor** — Maya Channel Box-style attribute editing with type-aware drag inputs, color pickers, and undoable transforms
 - **3D Viewport** — Hydra rendering via `UsdImagingGLEngine` into an offscreen FBO; switchable render delegates; grid overlay; selection bounding boxes; camera wireframes
-- **Camera Control** — Free orbital camera (tumble/truck/dolly) or drive from USD camera prims; auto near/far clipping; frame selection
+- **Multi-Viewport** — Single, horizontal split, vertical split, and quad layouts; per-tile camera and render settings; Space to maximize/restore
+- **Camera Control** — Free orbital camera (tumble/truck/dolly) or drive from USD camera prims; auto near/far clipping; frame selection; correct camera prim orientation for Y-up and Z-up stages
 - **Transform Gizmo** — Pure ImDrawList-based manipulator (translate/rotate/scale) in object or world space
-- **Undo/Redo** — Full command history for prim creation/deletion, attribute edits, transform changes, and layer operations
+- **Undo/Redo** — Full command history for prim creation/deletion, attribute edits, transform changes, and layer operations; Ctrl+Z / Ctrl+Y hotkeys; Edit menu integration
 - **Multi-select** — Rectangular selection in viewport with cross-panel syncing
 
 ## Dependencies
@@ -59,15 +60,23 @@ src/
 ├── core/                     — USD business logic (no UI dependency)
 │   ├── UsdStageManager       — Stage open/create/save/close lifecycle
 │   ├── LayerManager          — Layer stack introspection and mutation
-│   ├── PropertyManager       — Property read/write via edit target
+│   ├── PropertyManager       — Property read/write via edit target with undo
 │   ├── CommandHistory        — Undo/redo stack for ICommand instances
 │   ├── UsdSceneRenderer      — Hydra rendering + picking + overlays
 │   ├── ViewportCamera        — Free / USD-camera-driven orbital camera
 │   └── commands/             — ICommand subclasses for all mutable operations
+│       ├── TransformCommand      — Undoable translate/rotate/scale
+│       ├── CreatePrimCommand     — Undoable prim creation (with camera orientation)
+│       ├── DeletePrimCommand     — Undoable prim deletion (SdfCopySpec snapshot)
+│       ├── AttributeSetCommand   — Type-agnostic undoable attribute writes
+│       ├── AddReferenceCommand   — Undoable reference addition
+│       ├── ReplaceReferenceCommand — Undoable reference replacement
+│       └── LayerCommands         — Create, remove, reorder sublayers
 ├── ui/                       — ImGui panels and application shell
 │   ├── Application           — App shell (owns managers/panels, docking, menus)
-│   ├── ImGuiContext          — Win32+OpenGL+ImGui initialization and loop
-│   ├── ViewportPanel         — 3D viewport with toolbar, picking, camera
+│   ├── ImGuiContext           — Win32+OpenGL+ImGui initialization and loop
+│   ├── ViewportPanel         — Viewport container (layout, divider drag, maximize)
+│   ├── ViewportTile          — Single viewport tile (camera, rendering, overlays, picking)
 │   ├── SceneHierarchyPanel   — Prim tree browser with context menus
 │   ├── PropertyPanel         — Attribute/transform editor (channel box-style)
 │   ├── LayerPanel            — Layer stack editor with modal dialogs