diff --git a/quantiphy/quantiphy.py b/quantiphy/quantiphy.py
index 5066bf3..2339a50 100644
--- a/quantiphy/quantiphy.py
+++ b/quantiphy/quantiphy.py
@@ -418,18 +418,26 @@ def add_constant(value, alias=None, unit_systems=None):
 # These mappings are only used when writing numbers
 BIG_SCALE_FACTORS = 'kMGTPEZYRQ'
     # These must be given in order, one for every three decades.
-    # Use k rather than K, because K looks like a temperature when used alone.
+    # Use k rather than K because K looks like a temperature when used alone.
 
 SMALL_SCALE_FACTORS = 'munpfazyrq'
     # These must be given in order, one for every three decades.
 
-# Supported currency symbols (these go on left side of number)
+# Supported currency symbols (these precede the number)
 CURRENCY_SYMBOLS = '$€¥£₩₺₽₹Ƀ₿Ξ'
 
+# Units that abut the number.
+# % is controversial, NIST and ISO say that a space should be used to separate
+# the percent sign from a number, but the Chicago Manual of Style says the
+# opposite.
+TIGHT_UNITS = '''°'"′″'''
+    # The code is written assuming that TIGHT_UNITS includes only single
+    # character symbols, though the user can add multi-character tight units.
+
 # Unit symbols that are not simple letters.
 # Do not include % as it will be picked up when converting text to numbers,
 # which is generally not desired (you would end up converting 0.001% to 1m%).
-UNIT_SYMBOLS = '°ÅΩƱΩ℧¢$€¥£₩₺₽₹Ƀ₿șΞΔ'
+UNIT_SYMBOLS = """ÅΩƱΩ℧Δ¢ș""" + CURRENCY_SYMBOLS + TIGHT_UNITS
 
 # Regular expression for recognizing and decomposing string .format method codes
 FORMAT_SPEC = re.compile(r'''\A
@@ -494,7 +502,7 @@ def add_constant(value, alias=None, unit_systems=None):
     spacer = ' ',
     strip_radix = True,
     strip_zeros = True,
-    tight_units = ''' % ° ' " ′ ″ '''.split(),
+    tight_units = list(TIGHT_UNITS),
     unity_sf = '',
 )
 
@@ -977,9 +985,9 @@ def set_prefs(cls, **kwargs):
             to be '' or ' '; use the latter if you prefer a space between the
             number and the units. Generally using ' ' makes numbers easier to
             read, particularly with complex units, and using '' is easier to
-            parse.  You could also use a Unicode non-breaking space ' '. For
-            your convenience, you can access a non-breaking space using
-            :attr:`Quantity.non_breaking_space`,
+            parse.  Use of a non-breaking space is preferred when embedding
+            numbers in prose.  For your convenience, you can access a
+            non-breaking spaces using :attr:`Quantity.non_breaking_space`,
             :attr:`Quantity.narrow_non_breaking_space`, or
             :attr:`Quantity.thin_space`.
 
diff --git a/tests/test_misc.py b/tests/test_misc.py
index 829cfa6..32d28c4 100644
--- a/tests/test_misc.py
+++ b/tests/test_misc.py
@@ -179,6 +179,12 @@ def test_misc():
     q = Quantity('90°')
     assert q.render() == '90°'
 
+    q = Quantity("36 ′")
+    assert q.render() == "36′"
+
+    q = Quantity('18 ″')
+    assert q.render() == '18″'
+
     q = Quantity('80°F')
     assert q.render() == '80 °F'
 
@@ -304,7 +310,7 @@ class Foo(Quantity):
     q4 = Quantity('10%', name='bar', desc='buzz')
     assert '{:G}'.format(q3) == 'foo = 0.1'
     assert '{:G}'.format(q4) == 'bar = 10  # buzz'
-    assert '{:S}'.format(q4) == 'bar = 10%  # buzz'
+    assert '{:S}'.format(q4) == 'bar = 10 %  # buzz'
 
     class Derived(Quantity):
         pass
@@ -464,6 +470,8 @@ class Foo(Quantity):
     assert Quantity('10cm').render() == '100mm'
 
     Quantity.set_prefs(unity_sf='', spacer=' ')
+    assert Quantity('10%').render() == '10 %'
+    Quantity.set_prefs(tight_units='%')
     assert Quantity('10%').render() == '10%'
     Quantity.set_prefs(input_sf='%')
     assert Quantity('10%').render() == '100m'
@@ -1085,9 +1093,14 @@ def test_percent():
     # by default QuantiPhy treats % as a tight unit
 
     assert '%' not in Quantity.get_pref('input_sf')
-    assert Quantity('10%').render() == '10%'
+    assert Quantity('10%').render() == '10 %'
     assert Quantity('10%Δ').render() == '10 %Δ'
 
+    # adding % to tight_units
+    with Quantity.prefs(tight_units = Quantity.get_pref('tight_units') + ['%']):
+        assert Quantity('10%').render() == '10%'
+        assert Quantity('10%Δ').render() == '10 %Δ'
+
     # but by adding it to input_sf it is treated as a scale factor
     with Quantity.prefs(input_sf = Quantity.get_pref('input_sf') + '%'):
         assert '%' in Quantity.get_pref('input_sf')