diff --git a/cheuph/element.py b/cheuph/element.py
new file mode 100644
index 0000000..f192a9a
--- /dev/null
+++ b/cheuph/element.py
@@ -0,0 +1,17 @@
+from dataclasses import dataclass
+from typing import Hashable, Optional
+
+__all__ = ["Id", "Element", "RenderedElement"]
+
+Id = Hashable
+
+@dataclass
+class Element:
+    id: Id
+    parent_id: Optional[Id]
+
+@dataclass
+class RenderedElement:
+    id: Id
+    meta: AttributedText
+    lines: List[AttributedText]
diff --git a/cheuph/element_renderer.py b/cheuph/element_renderer.py
new file mode 100644
index 0000000..0af5616
--- /dev/null
+++ b/cheuph/element_renderer.py
@@ -0,0 +1,18 @@
+from abc import ABC, abstractmethod
+from typing import Generic, List, TypeVar
+
+from .element import Element, RenderedElement, MetaRenderedElement
+from .markup import AttributedText
+
+__all__ = ["ElementRenderer"]
+
+E = TypeVar("E", bound=Element)
+
+class ElementRenderer(ABC, Generic[E]):
+    """
+    Knows how to render elements.
+    """
+
+    @abstractmethod
+    def render(self, element: E, width: int) -> RenderedElement:
+        pass
diff --git a/cheuph/element_supply.py b/cheuph/element_supply.py
new file mode 100644
index 0000000..c427225
--- /dev/null
+++ b/cheuph/element_supply.py
@@ -0,0 +1,194 @@
+from abc import ABC, abstractmethod
+from typing import Dict, Generic, List, Optional, TypeVar
+
+from .element import Element, Id
+
+__all__ = ["ElementSupplyException", "ElementSupply", "InMemorySupply"]
+
+class ElementSupplyException(Exception):
+    pass
+
+E = TypeVar("E", bound=Element)
+
+class ElementSupply(ABC, Generic[E]):
+    """
+    An ElementSupply holds all of a room's known messages. It can be queried in
+    different ways. Messages can also be added or removed from the supply as
+    they are recived by the client.
+
+    For example, a supply could store messages in memory, use a sqlite backend,
+    or provide a caching layer above another supply.
+
+    Naming convention:
+
+    - All function parameters than end in "_id" are the id of an element
+    - All functions that end in "_id" return the id of the element they're
+      describing.
+    - All functions that end in "_ids" return a list of ids of the elements
+      they're describing.
+    """
+
+    @abstractmethod
+    def get(self, elem_id: Id) -> E:
+        """
+        Retrieve a message by is id.
+        """
+
+        pass
+
+    @abstractmethod
+    def parent_id(self, elem_id: Id) -> Optional[Id]:
+        """
+        Retrieve an element's parent.
+
+        Returns None if no parent exists.
+        """
+
+        pass
+
+    @abstractmethod
+    def child_ids(self, elem_id: Id) -> List[Id]:
+        """
+        Retrieve an element's children.
+        """
+
+        pass
+
+    @abstractmethod
+    def sibling_ids(self, elem_id: Id) -> List[Id]:
+        """
+        Retrieve an element's siblings.
+
+        The list must always contain at least the id of the query element.
+        """
+
+        pass
+
+    @abstractmethod
+    def lowest_root_id(self) -> Optional[Id]:
+        """
+        Retrieve the lowest root element.
+
+        A root element has no parent.
+        """
+
+        pass
+
+    def root_id(self, elem_id: Id) -> Id:
+        """
+        Find the root of the tree that an element is contained in.
+        """
+
+        ancestor_id = elem_id
+
+        while True:
+            parent_id = self.parent_id(ancestor_id)
+            if parent_id is None: break
+            ancestor_id = parent_id
+
+        return ancestor_id
+
+    def previous_id(self, elem_id: Id) -> Optional[Id]:
+        """
+        Find an element's previous (upper) sibling.
+        """
+
+        sibling_ids = self.sibling_ids(elem_id)
+
+        try:
+            index = sibling_ids.index(elem_id)
+            if index <= 0:
+                return None
+            else:
+                return sibling_ids[index - 1]
+        except ValueError:
+            return None
+
+    def next_id(self, elem_id: Id) -> Optional[Id]:
+        """
+        Find an element's next (lower) sibling.
+        """
+
+        sibling_ids = self.sibling_ids(elem_id)
+
+        try:
+            index = sibling_ids.index(elem_id)
+            if index >= len(sibling_ids) - 1:
+                return None
+            else:
+                return sibling_ids[index + 1]
+        except ValueError:
+            return None
+
+class InMemorySupply(ElementSupply[E]):
+    """
+    This supply stores messages in memory. It orders the messages by their ids.
+    """
+
+    def __init__(self) -> None:
+        self._elements: Dict[Id, E] = {}
+        self._children: Dict[Id, List[Id]] = {}
+
+    def add(self, elem: E) -> None:
+        if elem.id in self._elements:
+            self.remove(elem.id)
+
+        self._elements[elem.id] = elem
+
+        if elem.parent_id is not None:
+            children = self._children.get(elem.parent_id, [])
+            children.append(elem.id)
+            children.sort()
+            self._children[elem.parent_id] = children
+
+    def remove(self, elem_id: Id) -> None:
+        elem = self._elements.get(elem_id)
+        if elem is None: return
+
+        self._elements.pop(elem.id)
+
+        if elem.parent_id is not None:
+            children = self._children.get(elem.id)
+
+            if children is not None: # just to satisfy mypy
+                children.remove(elem)
+
+                if not children:
+                    self._children.pop(elem.id)
+
+    def get(self, elem_id: Id) -> E:
+        elem = self._elements.get(elem_id)
+
+        if elem is None:
+            raise ElementSupplyException(f"no element with id {elem_id!r}")
+
+        return elem
+
+    def child_ids(self, elem_id: Id) -> List[Id]:
+        self.get(elem_id) # Throw exception if element doesn't exist
+
+        return list(self._children.get(elem_id, []))
+
+    def parent_id(self, elem_id: Id) -> Optional[Id]:
+        elem = self.get(elem_id)
+        return elem.parent_id
+
+    def _roots(self) -> List[Id]:
+        roots = (m for m in self._elements.values() if m.parent_id is None)
+        return list(sorted(roots))
+
+    def sibling_ids(self, elem_id: Id) -> List[Id]:
+        parent_id = self.parent_id(elem_id)
+
+        if parent_id is None:
+            return self._roots()
+        else:
+            return self.child_ids(parent_id)
+
+    def lowest_root_id(self) -> Optional[Id]:
+        roots = self._roots()
+
+        if roots:
+            return roots[:-1]
+        else:
+            return None
diff --git a/cheuph/element_tree_renderer.py b/cheuph/element_tree_renderer.py
new file mode 100644
index 0000000..6209a0a
--- /dev/null
+++ b/cheuph/element_tree_renderer.py
@@ -0,0 +1,32 @@
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+from typing import Generic, TypeVar
+
+from .attributed_lines import AttributedLines
+from .element import Element, Id
+from .element_renderer import ElementRenderer
+from .element_supply import ElementSupply
+
+__all__ = ["ElementTreeRenderer", "CursorTreeRenderer"]
+
+E = TypeVar("E", bound=Element)
+
+class ElementTreeRenderer(ABC, Generic[E]):
+    def __init__(self,
+            supply: ElementSupply[E],
+            renderer: ElementRenderer[E],
+            ) -> None:
+        self.supply = supply
+        self.renderer = renderer
+
+        self.lines = AttributedLines()
+
+    @abstractmethod
+    def rerender(self) -> None:
+        pass
+
+class CursorTreeRenderer(ElementTreeRenderer[E]):
+    pass # TODO
+
+#class HighlightTreeRenderer(ElementTreeRenderer[E]):
+#    pass
diff --git a/cheuph/rendered_element_cache.py b/cheuph/rendered_element_cache.py
new file mode 100644
index 0000000..90bf71a
--- /dev/null
+++ b/cheuph/rendered_element_cache.py
@@ -0,0 +1,4 @@
+__all__ = ["RenderedElementCache"]
+
+class RenderedElementCache:
+    pass # TODO