diff --git a/chatmastermind/tags.py b/chatmastermind/tags.py
index bfe5fd5..544270c 100644
--- a/chatmastermind/tags.py
+++ b/chatmastermind/tags.py
@@ -1,7 +1,7 @@
 """
 Module implementing tag related functions and classes.
 """
-from typing import Type, TypeVar, Optional
+from typing import Type, TypeVar, Optional, Final
 
 TagInst = TypeVar('TagInst', bound='Tag')
 TagLineInst = TypeVar('TagLineInst', bound='TagLine')
@@ -16,9 +16,9 @@ class Tag(str):
     A single tag. A string that can contain anything but the default separator (' ').
     """
     # default separator
-    default_separator = ' '
+    default_separator: Final[str] = ' '
     # alternative separators (e. g. for backwards compatibility)
-    alternative_separators = [',']
+    alternative_separators: Final[list[str]] = [',']
 
     def __new__(cls: Type[TagInst], string: str) -> TagInst:
         """
@@ -93,19 +93,21 @@ def match_tags(tags: set[Tag], tags_or: Optional[set[Tag]], tags_and: Optional[s
 
 class TagLine(str):
     """
-    A line of tags. It starts with a prefix ('TAGS:'), followed by a list of tags,
-    separated by the defaut separator (' '). Any operations on a TagLine will sort
-    the tags.
+    A line of tags in a '.txt' file. It starts with a prefix ('TAGS:'), followed by
+    a list of tags, separated by the defaut separator (' '). Any operations on a
+    TagLine will sort the tags.
     """
     # the prefix
-    prefix = 'TAGS:'
+    prefix: Final[str] = 'TAGS:'
 
     def __new__(cls: Type[TagLineInst], string: str) -> TagLineInst:
         """
-        Make sure the tagline string starts with the prefix.
+        Make sure the tagline string starts with the prefix. Also replace newlines
+        and multiple spaces with ' ', in order to support multiline TagLines.
         """
         if not string.startswith(cls.prefix):
             raise TagError(f"TagLine '{string}' is missing prefix '{cls.prefix}'")
+        string = ' '.join(string.split())
         instance = super().__new__(cls, string)
         return instance
 
@@ -114,7 +116,7 @@ class TagLine(str):
         """
         Create a new TagLine from a set of tags.
         """
-        return cls(' '.join([TagLine.prefix] + sorted([t for t in tags])))
+        return cls(' '.join([cls.prefix] + sorted([t for t in tags])))
 
     def tags(self) -> set[Tag]:
         """
diff --git a/tests/test_main.py b/tests/test_main.py
index 8ce06cb..25cdc37 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -256,6 +256,10 @@ class TestTagLine(CmmTestCase):
         tagline = TagLine('TAGS: tag1 tag2')
         self.assertEqual(tagline, 'TAGS: tag1 tag2')
 
+    def test_valid_tagline_with_newline(self) -> None:
+        tagline = TagLine('TAGS: tag1\n tag2')
+        self.assertEqual(tagline, 'TAGS: tag1 tag2')
+
     def test_invalid_tagline(self) -> None:
         with self.assertRaises(TagError):
             TagLine('tag1 tag2')
@@ -273,6 +277,11 @@ class TestTagLine(CmmTestCase):
         tags = tagline.tags()
         self.assertEqual(tags, {Tag('tag1'), Tag('tag2')})
 
+    def test_tags_with_newline(self) -> None:
+        tagline = TagLine('TAGS: tag1\n tag2')
+        tags = tagline.tags()
+        self.assertEqual(tags, {Tag('tag1'), Tag('tag2')})
+
     def test_merge(self) -> None:
         tagline1 = TagLine('TAGS: tag1 tag2')
         tagline2 = TagLine('TAGS: tag2 tag3')