diff --git a/README.md b/README.md
index e195fa8..2e6e3fd 100644
--- a/README.md
+++ b/README.md
@@ -101,20 +101,20 @@ Use the `--join-lists` or `-jl` option to suppress linebreaks after list items:
 ```json
 {
     "bar": "baz",
-    "foo": [1,2,3]
+    "foo": [1, 2, 3]
 }
 ```
 Likewise, use the `--join-dict-items` or `-jd` option to suppress linebreaks after key/value pairs in a dict:
 ```json
-{"bar": "baz","foo": [
-        1,
-        2,
-        3
-    ]}
+{"bar": "baz", "foo": [
+    1,
+    2,
+    3
+]}
 ```
 Use `--condensed` or `-j` to apply both of these options:
 ```json
-{"bar": "baz","foo": [1,2,3]}
+{"bar": "baz", "foo": [1, 2, 3]}
 ```
 
 The `--only-edits` or `-e` option will print out a list of edits rather than applying them to the input file in place.
diff --git a/graphtage/json.py b/graphtage/json.py
index 80c2724..c5bd639 100644
--- a/graphtage/json.py
+++ b/graphtage/json.py
@@ -109,9 +109,34 @@ def __init__(self):
         """
         super().__init__('[', ']', ',')
 
+    def _joined(self, printer: Printer) -> bool:
+        return bool(getattr(printer, 'join_lists', False))
+
     def item_newline(self, printer: Printer, is_first: bool = False, is_last: bool = False):
-        if not hasattr(printer, 'join_lists') or not printer.join_lists:
+        """Separates two list items.
+
+        A joined list keeps all of its items on one line, separated by a single space so that the result reads as
+        ``[1, 2, 3]`` rather than ``[1,2,3]``.
+
+        """
+        if not self._joined(printer):
             printer.newline()
+        elif not is_first and not is_last:
+            printer.write(' ')
+
+    def items_indent(self, printer: Printer) -> Printer:
+        """Returns the printer context in which the list items are printed.
+
+        A joined list emits no newlines of its own, so indenting its items would have no visible effect on the list
+        itself while still adding a level of indentation to any nested sequence that *does* break across lines.
+
+        Returns:
+            Printer: :obj:`printer` itself if the list is joined, otherwise ``printer.indent()``.
+
+        """
+        if self._joined(printer):
+            return printer
+        return printer.indent()
 
     def print_ListNode(self, *args, **kwargs):
         """Prints a :class:`graphtage.ListNode`.
@@ -146,9 +171,34 @@ class JSONDictFormatter(SequenceFormatter):
     def __init__(self):
         super().__init__('{', '}', ',')
 
+    def _joined(self, printer: Printer) -> bool:
+        return bool(getattr(printer, 'join_dict_items', False))
+
     def item_newline(self, printer: Printer, is_first: bool = False, is_last: bool = False):
-        if not hasattr(printer, 'join_dict_items') or not printer.join_dict_items:
+        """Separates two dict items.
+
+        A joined dict keeps all of its items on one line, separated by a single space so that the result reads as
+        ``{"a": 1, "b": 2}`` rather than ``{"a": 1,"b": 2}``.
+
+        """
+        if not self._joined(printer):
             printer.newline()
+        elif not is_first and not is_last:
+            printer.write(' ')
+
+    def items_indent(self, printer: Printer) -> Printer:
+        """Returns the printer context in which the dict items are printed.
+
+        A joined dict emits no newlines of its own, so indenting its items would have no visible effect on the dict
+        itself while still adding a level of indentation to any nested sequence that *does* break across lines.
+
+        Returns:
+            Printer: :obj:`printer` itself if the dict is joined, otherwise ``printer.indent()``.
+
+        """
+        if self._joined(printer):
+            return printer
+        return printer.indent()
 
     def print_MultiSetNode(self, *args, **kwargs):
         """Prints a :class:`graphtage.MultiSetNode`.
diff --git a/test/test_json.py b/test/test_json.py
new file mode 100644
index 0000000..ef82f12
--- /dev/null
+++ b/test/test_json.py
@@ -0,0 +1,92 @@
+from io import StringIO
+from unittest import TestCase
+
+import graphtage
+from graphtage.printer import Printer
+from graphtage.utils import Tempfile
+
+NESTED = b'{"foo": [1, 2, 3], "bar": "baz"}'
+DEEP = b'{"x": [{"y": [1, 2]}, 3]}'
+
+
+def build(content: bytes) -> graphtage.TreeNode:
+    with Tempfile(content) as path:
+        return graphtage.FILETYPES_BY_TYPENAME["json"].build_tree(path)
+
+
+def render(from_content: bytes, to_content: bytes | None = None, *,
+           join_lists: bool = False, join_dict_items: bool = False) -> str:
+    """Renders the diff of two JSON documents with the given join options.
+
+    Args:
+        from_content: The JSON source of the "from" document.
+        to_content: The JSON source of the "to" document, or :const:`None` to diff ``from_content`` against itself.
+        join_lists: Whether to enable the ``--join-lists`` option.
+        join_dict_items: Whether to enable the ``--join-dict-items`` option.
+
+    Returns:
+        str: The rendered diff.
+
+    """
+    if to_content is None:
+        to_content = from_content
+    stream = StringIO()
+    printer = Printer(
+        out_stream=stream,
+        ansi_color=False,
+        quiet=True,
+        options={"join_lists": join_lists, "join_dict_items": join_dict_items},
+    )
+    diff = build(from_content).diff(build(to_content))
+    graphtage.FILETYPES_BY_TYPENAME["json"].get_default_formatter().print(printer, diff)
+    printer.flush(final=True)
+    return stream.getvalue()
+
+
+class TestJSONJoinOptions(TestCase):
+    """Covers ``--join-lists`` and ``--join-dict-items``, which previously had no test coverage at all."""
+
+    def test_default_output_is_pretty_printed(self):
+        self.assertEqual(
+            '{\n    "bar": "baz",\n    "foo": [\n        1,\n        2,\n        3\n    ]\n}',
+            render(NESTED)
+        )
+
+    def test_join_lists_keeps_the_dict_pretty_printed(self):
+        self.assertEqual('{\n    "bar": "baz",\n    "foo": [1, 2, 3]\n}', render(NESTED, join_lists=True))
+
+    def test_join_dict_items_does_not_indent_a_nested_list(self):
+        """A joined dict used to leave an unused indent on the printer, which the nested list then doubled.
+
+        The list items came out eight spaces deep and the closing bracket four, as though the collapsed dict were
+        still occupying a level of indentation.
+
+        """
+        self.assertEqual('{"bar": "baz", "foo": [\n    1,\n    2,\n    3\n]}', render(NESTED, join_dict_items=True))
+
+    def test_condensed_joins_both_lists_and_dicts(self):
+        self.assertEqual(
+            '{"bar": "baz", "foo": [1, 2, 3]}',
+            render(NESTED, join_lists=True, join_dict_items=True)
+        )
+
+    def test_join_dict_items_indents_by_the_depth_that_breaks_lines(self):
+        """Only the sequences that actually emit newlines contribute a level of indentation."""
+        self.assertEqual(
+            '{"x": [\n    {"y": [\n        1,\n        2\n    ]},\n    3\n]}',
+            render(DEEP, join_dict_items=True)
+        )
+
+    def test_join_lists_indents_by_the_depth_that_breaks_lines(self):
+        self.assertEqual('{\n    "x": [{\n        "y": [1, 2]\n    }, 3]\n}', render(DEEP, join_lists=True))
+
+    def test_joined_items_are_separated_by_a_space(self):
+        """Joined output used to run the items together as ``[1,2,3]`` and ``{"bar": "baz","foo": …}``."""
+        self.assertIn('[1, 2, 3]', render(NESTED, join_lists=True))
+        self.assertIn('"baz", "foo"', render(NESTED, join_dict_items=True))
+
+    def test_joined_output_separates_inserted_and_removed_items(self):
+        self.assertEqual(
+            '{"a": [1, ~~2~~, 3], ++"b": 4++}',
+            render(b'{"a": [1, 2, 3]}', b'{"a": [1, 3], "b": 4}', join_lists=True, join_dict_items=True)
+        )