diff --git a/index/__init__.py b/index/__init__.py
index 4f78262..e00b36d 100644
--- a/index/__init__.py
+++ b/index/__init__.py
@@ -90,6 +90,12 @@ def click_add_metadata(metadata_type, lemma, lemma_type, md_value, overwrite):
 def click_find_ref(search_term):
     find_ref(search_term)
 
+@click.command()
+@click.argument("lemma", type=str)
+@click.argument("alias", type=str)
+def click_add_alias(lemma, alias):
+    add_alias(lemma, alias)
+
 
 @click.command()
 def click_format_refs():
diff --git a/index/index.py b/index/index.py
index 6847c92..a018672 100644
--- a/index/index.py
+++ b/index/index.py
@@ -75,10 +75,29 @@ def add_ref(
         ref_dict["type"] = ref_type
 
     # Determine whether that lemma is already in the index
+    lemma_in_index = False
     try:
         lemma_entry = index[lemma]
     except KeyError:
         # This triggers if the lemma is not present in the index.
+
+        # Check whether lemma is an alias.
+        for k, v in index.items():
+            if "aliases" in v:
+                if lemma in v["aliases"]:
+                    click.echo(f"'{lemma}' is known to be an alias for '{k}' in this index. Reference will be added to '{k}'")
+                    lemma = k
+                    lemma_entry = index[lemma]
+                    lemma_in_index = True
+                    break
+                else:
+                    continue
+            else:
+                continue
+    else:
+        lemma_in_index = True
+
+    if not lemma_in_index:
         lemma_entry = {"references": {work: [ref_dict]}}
         if lemma_type:
             lemma_entry["type"] = lemma_type
@@ -216,6 +235,39 @@ def find_ref(search_term, max_l_dist=2, num_results=5):
         print("No matches found")
 
 
+def add_alias(lemma: str, alias: str):
+    """Add an alias to a lemma in the index. An alias is regarded as a property of the lemma."""
+
+    with open(INDEX_FILE, encoding='utf-8') as f:
+        index = yaml.safe_load(f)
+    
+    if lemma not in index:
+        raise click.BadParameter(f"Lemma '{lemma}' not found in index.")
+
+    if alias in index:
+        raise click.BadParameter(f"Alias '{alias}' is a lemma in the index. Please use 'add-rel' to add it as a relation between lemmata.")
+
+    # Check whether lemma is an alias.
+    for k, v in index.items():
+        if "aliases" in v:
+            if lemma in v["aliases"]:
+                raise click.BadParameter(f"Alias '{alias}' is already an alias for '{k}' in the index. No duplicates are allowed.")
+                break
+            else:
+                continue
+        else:
+            continue
+
+    lemma_dict = index[lemma]
+
+    index[lemma]["aliases"] = lemma_dict.get("aliases", []) + [
+        alias
+    ]
+
+    with open(INDEX_FILE, "w", encoding='utf-8') as f:
+        yaml.safe_dump(index, f, allow_unicode=True)
+
+
 def add_metadata(md_type, lemma, lemma_type, md_value=None, overwrite=False):
     """Add metadata to a lemma of type 'author' or 'work'"""
 
diff --git a/setup.py b/setup.py
index 984e479..08af3ab 100644
--- a/setup.py
+++ b/setup.py
@@ -11,6 +11,7 @@
             "add-ref=index:click_add_ref",
             "add-rel=index:click_add_rel",
             "add-metadata=index:click_add_metadata",
+            "add-alias=index:click_add_alias",
             "find-ref=index:click_find_ref",
             "format-refs=index:click_format_refs",
         ]
diff --git a/validation/schema.yml b/validation/schema.yml
index b4548b2..0eb6136 100644
--- a/validation/schema.yml
+++ b/validation/schema.yml
@@ -4,6 +4,12 @@ additionalProperties:
   type: object
   description: Lemma object
   properties:
+    aliases:
+      type: array
+      description: List of aliases of lemma.
+      uniqueItems: True
+      items:
+        type: string
     metadata:
       type: object
       description: Metadata object