diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0797decb5..981f49e0c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,18 +7,22 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12"]
         numpy: [null, "numpy>=1.23,<2.0.0", "numpy>=2.0.0rc1"]
         uncertainties: [null, "uncertainties==3.1.6", "uncertainties>=3.1.6,<4.0.0"]
         extras: [null]
         include:
           - python-version: "3.10" # Minimal versions
-            numpy: "numpy"
-            extras: matplotlib==2.2.5
+            numpy: "numpy>=1.23,<2.0.0"
+            extras: matplotlib==3.5.3
           - python-version: "3.10"
             numpy: "numpy"
             uncertainties: "uncertainties"
-            extras: "sparse xarray netCDF4 dask[complete]==2023.4.0 graphviz babel==2.8 mip>=1.13"
+            extras: "sparse xarray netCDF4 dask[complete]==2024.5.1 graphviz babel==2.8 mip>=1.13"
+          - python-version: "3.10"
+            numpy: "numpy==1.26.1"
+            uncertainties: null
+            extras: "babel==2.15 matplotlib==3.9.0"
     runs-on: ubuntu-latest
 
     env:
diff --git a/CHANGES b/CHANGES
index e2ad2775a..208dbad06 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4,7 +4,18 @@ Pint Changelog
 0.25 (unreleased)
 -----------------
 
-Added mu and mc as alternatives for SI micro prefix
+- Added mu and mc as alternatives for SI micro prefix
+- Added ℓ as alternative for liter
+- Support permille units and `‰` symbol (PR #2033, Issue #1963)
+
+
+0.24.1 (2024-06-24)
+-----------------
+
+- Fix custom formatter needing the registry object. (PR #2011)
+- Support python 3.9 following difficulties installing with NumPy 2. (PR #2019)
+- Fix default formatting of dimensionless unit issue. (PR #2012)
+- Fix bug preventing custom formatters with modifiers working. (PR #2021)
 
 
 0.24 (2024-06-07)
@@ -65,6 +76,7 @@ Added mu and mc as alternatives for SI micro prefix
 - Add numpy.linalg.norm implementation.
   (PR #1251)
 
+
 0.22 (2023-05-25)
 -----------------
 
diff --git a/docs/_static/style.css b/docs/_static/style.css
index b2bc297d6..a2ac3f7fd 100644
--- a/docs/_static/style.css
+++ b/docs/_static/style.css
@@ -38,8 +38,14 @@ pre, code {
 
 .sd-card .sd-card-header {
   border: none;
-  color: #150458 !important;
+  color: #150458;
   font-size: var(--pst-font-size-h5);
   font-weight: bold;
   padding: 2.5rem 0rem 0.5rem 0rem;
 }
+
+html[data-theme=dark] {
+  .sd-card .sd-card-header {
+    color: #FFF;
+  }
+}
diff --git a/docs/advanced/currencies.rst b/docs/advanced/currencies.rst
index 26b66b531..addc94785 100644
--- a/docs/advanced/currencies.rst
+++ b/docs/advanced/currencies.rst
@@ -84,3 +84,16 @@ currency on its own dimension, and then implement transformations::
 More sophisticated formulas, e.g. dealing with flat fees and thresholds, can be
 implemented with arbitrary python code by programmatically defining a context (see
 :ref:`contexts`).
+
+Currency Symbols
+----------------
+
+Many common currency symbols are not supported by the pint parser. A preprocessor can be used as a workaround:
+
+.. doctest::
+
+   >>> import pint
+   >>> ureg = pint.UnitRegistry(preprocessors = [lambda s: s.replace("€", "EUR")])
+   >>> ureg.define("euro = [currency] = € = EUR")
+   >>> print(ureg.Quantity("1 €"))
+   1 euro
diff --git a/docs/user/formatting.rst b/docs/user/formatting.rst
index d45fc1e13..fbf2fae42 100644
--- a/docs/user/formatting.rst
+++ b/docs/user/formatting.rst
@@ -98,7 +98,8 @@ formats:
    '2.3e-06 kilogram ** -1 * meter ** 3 * second ** -2'
 
 where ``unit`` is a :py:class:`dict` subclass containing the unit names and
-their exponents.
+their exponents, ``registry`` is the current instance of :py:class:``UnitRegistry`` and
+``options`` is not yet implemented.
 
 You can choose to replace the complete formatter. Briefly, the formatter if an object with the
 following methods: `format_magnitude`, `format_unit`, `format_quantity`, `format_uncertainty`,
diff --git a/docs/user/log_units.rst b/docs/user/log_units.rst
index 03e007914..096397350 100644
--- a/docs/user/log_units.rst
+++ b/docs/user/log_units.rst
@@ -111,16 +111,16 @@ will not work:
 .. doctest::
 
     >>> -161.0 * ureg('dBm/Hz') == (-161.0 * ureg.dBm / ureg.Hz)
-    False
+    np.False_
 
 But this will:
 
 .. doctest::
 
     >>> ureg('-161.0 dBm/Hz') == (-161.0 * ureg.dBm / ureg.Hz)
-    True
+    np.True_
     >>> Q_(-161.0, 'dBm') / ureg.Hz == (-161.0 * ureg.dBm / ureg.Hz)
-    True
+    np.True_
 
 To begin using this feature while avoiding problems, define logarithmic units
 as single-unit quantities and convert them to their base units as quickly as
diff --git a/pint/compat.py b/pint/compat.py
index 277662410..b968803ce 100644
--- a/pint/compat.py
+++ b/pint/compat.py
@@ -19,9 +19,13 @@
 from typing import (
     Any,
     NoReturn,
-    TypeAlias,  # noqa
 )
 
+if sys.version_info >= (3, 10):
+    from typing import TypeAlias  # noqa
+else:
+    from typing_extensions import TypeAlias  # noqa
+
 if sys.version_info >= (3, 11):
     from typing import Self  # noqa
 else:
@@ -71,7 +75,8 @@ class BehaviorChangeWarning(UserWarning):
 
 try:
     from uncertainties import UFloat, ufloat
-    from uncertainties import unumpy as unp
+
+    unp = None
 
     HAS_UNCERTAINTIES = True
 except ImportError:
@@ -88,6 +93,8 @@ class BehaviorChangeWarning(UserWarning):
     HAS_NUMPY = True
     NUMPY_VER = np.__version__
     if HAS_UNCERTAINTIES:
+        from uncertainties import unumpy as unp
+
         NUMERIC_TYPES = (Number, Decimal, ndarray, np.number, UFloat)
     else:
         NUMERIC_TYPES = (Number, Decimal, ndarray, np.number)
diff --git a/pint/default_en.txt b/pint/default_en.txt
index 378517f0a..4250a48cb 100644
--- a/pint/default_en.txt
+++ b/pint/default_en.txt
@@ -150,6 +150,7 @@ byte = 8 * bit = B = octet
 
 # Ratios
 percent = 0.01 = %
+permille = 0.001 = ‰
 ppm = 1e-6
 
 # Length
@@ -221,7 +222,7 @@ hectare = 100 * are = ha
 
 # Volume
 [volume] = [length] ** 3
-liter = decimeter ** 3 = l = L = litre
+liter = decimeter ** 3 = l = L = ℓ = litre
 cubic_centimeter = centimeter ** 3 = cc
 lambda = microliter = λ
 stere = meter ** 3
diff --git a/pint/delegates/formatter/_compound_unit_helpers.py b/pint/delegates/formatter/_compound_unit_helpers.py
index 89bda87a2..06a8ac2d3 100644
--- a/pint/delegates/formatter/_compound_unit_helpers.py
+++ b/pint/delegates/formatter/_compound_unit_helpers.py
@@ -20,12 +20,11 @@
     TYPE_CHECKING,
     Any,
     Literal,
-    TypeAlias,
     TypedDict,
     TypeVar,
 )
 
-from ...compat import babel_parse
+from ...compat import TypeAlias, babel_parse
 from ...util import UnitsContainer
 
 T = TypeVar("T")
@@ -83,11 +82,19 @@ def localize_per(
     locale = babel_parse(locale)
 
     patterns = locale._data["compound_unit_patterns"].get("per", None)
+    if patterns is None:
+        return default or "{}/{}"
 
+    patterns = patterns.get(length, None)
     if patterns is None:
         return default or "{}/{}"
 
-    return patterns.get(length, default or "{}/{}")
+    # babel 2.8
+    if isinstance(patterns, str):
+        return patterns
+
+    # babe; 2.15
+    return patterns.get("compound", default or "{}/{}")
 
 
 @functools.lru_cache
@@ -257,6 +264,12 @@ def prepare_compount_unit(
 
     # out: unit_name, unit_exponent
 
+    if len(out) == 0:
+        if "~" in spec:
+            return ([], [])
+        else:
+            return ([("dimensionless", 1)], [])
+
     if "~" in spec:
         if registry is None:
             raise ValueError(
diff --git a/pint/delegates/formatter/_format_helpers.py b/pint/delegates/formatter/_format_helpers.py
index 995159e65..8a2f37a59 100644
--- a/pint/delegates/formatter/_format_helpers.py
+++ b/pint/delegates/formatter/_format_helpers.py
@@ -131,6 +131,8 @@ def join_mu(joint_fstring: str, mstr: str, ustr: str) -> str:
 
     This avoids that `3 and `1 / m` becomes `3 1 / m`
     """
+    if ustr == "":
+        return mstr
     if ustr.startswith("1 / "):
         return joint_fstring.format(mstr, ustr[2:])
     return joint_fstring.format(mstr, ustr)
diff --git a/pint/delegates/formatter/_to_register.py b/pint/delegates/formatter/_to_register.py
index 0f8f46788..697973716 100644
--- a/pint/delegates/formatter/_to_register.py
+++ b/pint/delegates/formatter/_to_register.py
@@ -61,6 +61,8 @@ def wrapper(func: Callable[[PlainUnit, UnitRegistry], str]):
             raise ValueError(f"format {name!r} already exists")  # or warn instead
 
         class NewFormatter(BaseFormatter):
+            spec = name
+
             def format_magnitude(
                 self,
                 magnitude: Magnitude,
diff --git a/pint/delegates/formatter/full.py b/pint/delegates/formatter/full.py
index e6d0eee47..d5de43326 100644
--- a/pint/delegates/formatter/full.py
+++ b/pint/delegates/formatter/full.py
@@ -102,12 +102,20 @@ def get_formatter(self, spec: str):
             if k in spec:
                 return v
 
-        try:
-            return REGISTERED_FORMATTERS[spec]
-        except KeyError:
-            pass
+        for k, v in REGISTERED_FORMATTERS.items():
+            if k in spec:
+                orphan_fmt = REGISTERED_FORMATTERS[k]
+                break
+        else:
+            return self._formatters["D"]
 
-        return self._formatters["D"]
+        try:
+            fmt = orphan_fmt.__class__(self._registry)
+            spec = getattr(fmt, "spec", spec)
+            self._formatters[spec] = fmt
+            return fmt
+        except Exception:
+            return orphan_fmt
 
     def format_magnitude(
         self, magnitude: Magnitude, mspec: str = "", **babel_kwds: Unpack[BabelKwds]
diff --git a/pint/facets/plain/registry.py b/pint/facets/plain/registry.py
index 09fd220ee..c5c0be783 100644
--- a/pint/facets/plain/registry.py
+++ b/pint/facets/plain/registry.py
@@ -255,6 +255,9 @@ def __init__(
         # use a default preprocessor to support "%"
         self.preprocessors.insert(0, lambda string: string.replace("%", " percent "))
 
+        # use a default preprocessor to support permille "‰"
+        self.preprocessors.insert(0, lambda string: string.replace("‰", " permille "))
+
         #: mode used to fill in the format defaults
         self.separate_format_defaults = separate_format_defaults
 
diff --git a/pint/testsuite/helpers.py b/pint/testsuite/helpers.py
index c9106b75a..d317e0755 100644
--- a/pint/testsuite/helpers.py
+++ b/pint/testsuite/helpers.py
@@ -128,9 +128,26 @@ def requires_numpy_at_least(version):
     )
 
 
-requires_babel = pytest.mark.skipif(
-    not HAS_BABEL, reason="Requires Babel with units support"
-)
+def requires_babel(tested_locales=[]):
+    if not HAS_BABEL:
+        return pytest.mark.skip("Requires Babel with units support")
+
+    import locale
+
+    default_locale = locale.getlocale(locale.LC_NUMERIC)
+    locales_unavailable = False
+    try:
+        for loc in tested_locales:
+            locale.setlocale(locale.LC_NUMERIC, loc)
+    except locale.Error:
+        locales_unavailable = True
+    locale.setlocale(locale.LC_NUMERIC, default_locale)
+
+    return pytest.mark.skipif(
+        locales_unavailable, reason="Tested locales not available."
+    )
+
+
 requires_not_babel = pytest.mark.skipif(
     HAS_BABEL, reason="Requires Babel not to be installed"
 )
diff --git a/pint/testsuite/test_babel.py b/pint/testsuite/test_babel.py
index 2dd66d58d..9adcb04a9 100644
--- a/pint/testsuite/test_babel.py
+++ b/pint/testsuite/test_babel.py
@@ -16,7 +16,7 @@ def test_no_babel(func_registry):
         distance.format_babel(locale="fr_FR", length="long")
 
 
-@helpers.requires_babel()
+@helpers.requires_babel(["fr_FR", "ro_RO"])
 def test_format(func_registry):
     ureg = func_registry
     dirname = os.path.dirname(__file__)
@@ -36,7 +36,7 @@ def test_format(func_registry):
     assert mks.format_babel(locale="fr_FR") == "métrique"
 
 
-@helpers.requires_babel()
+@helpers.requires_babel(["fr_FR", "ro_RO"])
 def test_registry_locale():
     ureg = UnitRegistry(fmt_locale="fr_FR")
     dirname = os.path.dirname(__file__)
@@ -60,7 +60,7 @@ def test_registry_locale():
     assert mks.format_babel(locale="fr_FR") == "métrique"
 
 
-@helpers.requires_babel()
+@helpers.requires_babel(["fr_FR"])
 def test_unit_format_babel():
     ureg = UnitRegistry(fmt_locale="fr_FR")
     volume = ureg.Unit("ml")
@@ -85,7 +85,7 @@ def test_no_registry_locale(func_registry):
         distance.format_babel()
 
 
-@helpers.requires_babel()
+@helpers.requires_babel(["fr_FR"])
 def test_str(func_registry):
     ureg = func_registry
     d = 24.1 * ureg.meter
diff --git a/pint/testsuite/test_formatting.py b/pint/testsuite/test_formatting.py
index e74c09c50..d8f10715b 100644
--- a/pint/testsuite/test_formatting.py
+++ b/pint/testsuite/test_formatting.py
@@ -59,6 +59,8 @@ def test_split_format(format, default, flag, expected):
 def test_register_unit_format(func_registry):
     @fmt.register_unit_format("custom")
     def format_custom(unit, registry, **options):
+        # Ensure the registry is correct..
+        registry.Unit(unit)
         return "<formatted unit>"
 
     quantity = 1.0 * func_registry.meter
diff --git a/pint/testsuite/test_issues.py b/pint/testsuite/test_issues.py
index 611adde33..847f269f0 100644
--- a/pint/testsuite/test_issues.py
+++ b/pint/testsuite/test_issues.py
@@ -7,7 +7,12 @@
 
 import pytest
 
-from pint import Context, DimensionalityError, UnitRegistry, get_application_registry
+from pint import (
+    Context,
+    DimensionalityError,
+    UnitRegistry,
+    get_application_registry,
+)
 from pint.compat import np
 from pint.delegates.formatter._compound_unit_helpers import sort_by_dimensionality
 from pint.facets.plain.unit import UnitsContainer
@@ -409,6 +414,9 @@ def test_micro_creation_mu(self, module_registry):
     def test_micro_creation_mc(self, module_registry):
         module_registry.Quantity(2, "mcg")
 
+    def test_liter_creation_U2113(self, module_registry):
+        module_registry.Quantity(2, "ℓ")
+
     @helpers.requires_numpy
     def test_issue171_real_imag(self, module_registry):
         qr = [1.0, 2.0, 3.0, 4.0] * module_registry.meter
@@ -885,6 +893,24 @@ def test_issue1277(self, module_registry):
         assert c.to("percent").m == 50
         # assert c.to("%").m == 50  # TODO: fails.
 
+    def test_issue1963(self, module_registry):
+        ureg = module_registry
+        assert ureg("‰") == ureg("permille")
+        assert ureg("‰") == ureg.permille
+
+        a = ureg.Quantity("10 ‰")
+        b = ureg.Quantity("100 ppm")
+        c = ureg.Quantity("0.5")
+
+        assert f"{a}" == "10 permille"
+        assert f"{a:~}" == "10 ‰"
+
+        assert_equal(a, 0.01)
+        assert_equal(1e2 * b, a)
+        assert_equal(c, 50 * a)
+
+        assert_equal((1 * ureg.milligram) / (1 * ureg.gram), ureg.permille)
+
     @pytest.mark.xfail
     @helpers.requires_uncertainties()
     def test_issue_1300(self):
@@ -914,7 +940,7 @@ def test_issue1674(self, module_registry):
             arr_of_q * q_arr, np.array([Q_(2, "m^2"), Q_(8, "m s")], dtype="object")
         )
 
-    @helpers.requires_babel()
+    @helpers.requires_babel(["es_ES"])
     def test_issue_1400(self, sess_registry):
         q1 = 3.1 * sess_registry.W
         q2 = 3.1 * sess_registry.W / sess_registry.cm
@@ -923,6 +949,7 @@ def test_issue_1400(self, sess_registry):
         assert q2.format_babel("~", locale="es_ES") == "3,1 W/cm"
         assert q2.format_babel("", locale="es_ES") == "3,1 vatios por centímetro"
 
+    @helpers.requires_numpy()
     @helpers.requires_uncertainties()
     def test_issue1611(self, module_registry):
         from numpy.testing import assert_almost_equal
@@ -1261,3 +1288,46 @@ def test_issue1949(registry_empty):
 def test_issue1772(given, expected):
     ureg = UnitRegistry(non_int_type=decimal.Decimal)
     assert f"{ureg(given):Lx}" == expected
+
+
+def test_issue2017():
+    ureg = UnitRegistry()
+
+    from pint import formatting as fmt
+
+    @fmt.register_unit_format("test")
+    def _test_format(unit, registry, **options):
+        print("format called")
+        proc = {u.replace("µ", "u"): e for u, e in unit.items()}
+        return fmt.formatter(
+            proc.items(),
+            as_ratio=True,
+            single_denominator=False,
+            product_fmt="*",
+            division_fmt="/",
+            power_fmt="{}{}",
+            parentheses_fmt="({})",
+            **options,
+        )
+
+    base_unit = ureg.microsecond
+    assert f"{base_unit:~test}" == "us"
+    assert f"{base_unit:test}" == "microsecond"
+
+
+def test_issue2007():
+    ureg = UnitRegistry()
+    q = ureg.Quantity(1, "")
+    assert f"{q:P}" == "1 dimensionless"
+    assert f"{q:C}" == "1 dimensionless"
+    assert f"{q:D}" == "1 dimensionless"
+    assert f"{q:H}" == "1 dimensionless"
+
+    assert f"{q:L}" == "1\\ \\mathrm{dimensionless}"
+    #  L returned '1\\ dimensionless' in pint 0.23
+
+    assert f"{q:Lx}" == "\\SI[]{1}{}"
+    assert f"{q:~P}" == "1"
+    assert f"{q:~C}" == "1"
+    assert f"{q:~D}" == "1"
+    assert f"{q:~H}" == "1"
diff --git a/pyproject.toml b/pyproject.toml
index a376bd6a4..9f29f8f92 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -22,11 +22,12 @@ classifiers = [
     "Programming Language :: Python",
     "Topic :: Scientific/Engineering",
     "Topic :: Software Development :: Libraries",
+    "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
 ]
-requires-python = ">=3.10"
+requires-python = ">=3.9"
 dynamic = ["version", "dependencies"]
 
 [tool.setuptools.package-data]