diff --git a/spec/std/sprintf_spec.cr b/spec/std/sprintf_spec.cr
index d3c1952f2726..1170a0ee5ea1 100644
--- a/spec/std/sprintf_spec.cr
+++ b/spec/std/sprintf_spec.cr
@@ -35,6 +35,24 @@ describe "::sprintf" do
     assert_sprintf "%%*%%", [1, 2, 3], "%*%"
   end
 
+  it "doesn't accept modifiers for %%" do
+    expect_raises(ArgumentError) { sprintf("%0%") }
+    expect_raises(ArgumentError) { sprintf("%+%") }
+    expect_raises(ArgumentError) { sprintf("%-%") }
+    expect_raises(ArgumentError) { sprintf("% %") }
+    expect_raises(ArgumentError) { sprintf("%#%") }
+    expect_raises(ArgumentError) { sprintf("%.0%") }
+    expect_raises(ArgumentError) { sprintf("%*%", 1) }
+
+    expect_raises(ArgumentError) { sprintf("%<a>0%") }
+    expect_raises(ArgumentError) { sprintf("%<a>+%") }
+    expect_raises(ArgumentError) { sprintf("%<a>-%") }
+    expect_raises(ArgumentError) { sprintf("%<a> %") }
+    expect_raises(ArgumentError) { sprintf("%<a>#%") }
+    expect_raises(ArgumentError) { sprintf("%<a>.0%") }
+    expect_raises(ArgumentError) { sprintf("%<a>*%", 1) }
+  end
+
   context "integers" do
     context "base specifier" do
       it "supports base 2" do
@@ -532,6 +550,13 @@ describe "::sprintf" do
       expect_raises(ArgumentError, "One hash or named tuple required") { sprintf("change %{this}", "this") }
     end
 
+    it "doesn't raise if 1-element list of hash or named tuple given" do
+      assert_sprintf "change %{this}", [{"this" => "nothing"}], "change nothing"
+      assert_sprintf "change %{this}", [{this: "nothing"}], "change nothing"
+      assert_sprintf "change %{this}", { {"this" => "nothing"} }, "change nothing"
+      assert_sprintf "change %{this}", { {this: "nothing"} }, "change nothing"
+    end
+
     it "raises on unbalanced curly" do
       expect_raises(ArgumentError, "Malformed name - unmatched parenthesis") { sprintf("change %{this", {"this" => 1}) }
     end
@@ -539,6 +564,14 @@ describe "::sprintf" do
     it "doesn't raise on balanced curly with null byte" do
       assert_sprintf "change %{this\u{0}}", {"this\u{0}" => 1}, "change 1"
     end
+
+    it "raises if sequential parameters also given" do
+      expect_raises(ArgumentError, "Cannot mix named parameters with sequential ones") { sprintf("%{this}%d", {"this" => 1}) }
+    end
+
+    it "doesn't raise if formatted substitution also given" do
+      assert_sprintf "%{foo}%<bar>s", {"foo" => "x", "bar" => "y"}, "xy"
+    end
   end
 
   context "formatted substitution" do
@@ -546,5 +579,20 @@ describe "::sprintf" do
       assert_sprintf "change %<this>.2f", {"this" => 23.456}, "change 23.46"
       assert_sprintf "change %<this>.2f", {this: 23.456}, "change 23.46"
     end
+
+    it "raises if sequential parameters also given" do
+      expect_raises(ArgumentError, "Cannot mix named parameters with sequential ones") { sprintf("%<this>d%d", {"this" => 1}) }
+    end
+
+    it "doesn't raise if plain substitution also given" do
+      assert_sprintf "%<foo>s%{bar}", {"foo" => "x", "bar" => "y"}, "xy"
+    end
+  end
+
+  context "sequential parameters" do
+    it "raises if named parameters also given" do
+      expect_raises(ArgumentError, "Cannot mix sequential parameters with named ones") { sprintf("%d%{this}", 1) }
+      expect_raises(ArgumentError, "Cannot mix sequential parameters with named ones") { sprintf("%d%<this>d", 1) }
+    end
   end
 end
diff --git a/src/kernel.cr b/src/kernel.cr
index efa06b3d6676..6653536f285c 100644
--- a/src/kernel.cr
+++ b/src/kernel.cr
@@ -166,6 +166,9 @@ end
 # %<name>[flags][width][.precision]type
 # ```
 #
+# The percent sign itself is escaped by `%%`. No format modifiers are allowed,
+# and no arguments are consumed.
+#
 # The field type controls how the corresponding argument value is to be
 # interpreted, while the flags modify that interpretation.
 #
@@ -208,7 +211,6 @@ end
 #   s   | Argument is a string to be substituted. If the format
 #       | sequence contains a precision, at most that many characters
 #       | will be copied.
-#   %   | A percent sign itself will be displayed. No argument taken.
 # ```
 #
 # Flags modify the behavior of the format specifiers:
diff --git a/src/string/formatter.cr b/src/string/formatter.cr
index aecdd1409178..eb9982a471af 100644
--- a/src/string/formatter.cr
+++ b/src/string/formatter.cr
@@ -2,8 +2,19 @@ require "c/stdio"
 
 # :nodoc:
 struct String::Formatter(A)
+  private enum Mode
+    None
+
+    # `%s`, index type is `Nil`
+    Sequential
+
+    # `%{a}` or `%<b>s`, index type is `String`
+    Named
+  end
+
   @format_buf : Pointer(UInt8)?
   @temp_buf : Pointer(UInt8)?
+  @arg_mode : Mode = :none
 
   def initialize(string, @args : A, @io : IO)
     @reader = Char::Reader.new(string)
@@ -18,7 +29,7 @@ struct String::Formatter(A)
       when '%'
         consume_percent
       else
-        char char
+        @io << char
       end
       next_char
     end
@@ -32,33 +43,27 @@ struct String::Formatter(A)
     when '<'
       next_char
       consume_formatted_substitution
+    when '%'
+      @io << '%'
     else
       flags = consume_flags
-      consume_type flags
+      consume_type flags, nil
     end
   end
 
   private def consume_substitution
     key = consume_substitution_key '}'
-    arg = current_arg
-    if arg.is_a?(Hash) || arg.is_a?(NamedTuple)
-      @io << arg[key]
-    else
-      raise ArgumentError.new "One hash or named tuple required"
-    end
+    # note: "`@io << (arg_at(key))` has no type" without this `arg` variable
+    arg = arg_at(key)
+    @io << arg
   end
 
   private def consume_formatted_substitution
     key = consume_substitution_key '>'
+    args_are :named
     next_char
-    arg = current_arg
-    if arg.is_a?(Hash) || arg.is_a?(NamedTuple)
-      target_arg = arg[key]
-    else
-      raise ArgumentError.new "One hash or named tuple required"
-    end
     flags = consume_flags
-    consume_type flags, target_arg, true
+    consume_type flags, key
   end
 
   private def consume_substitution_key(end_char)
@@ -146,10 +151,9 @@ struct String::Formatter(A)
   end
 
   private def consume_dynamic_value
-    value = current_arg
+    value = arg_at(nil)
     if value.is_a?(Int)
       next_char
-      next_arg
       value.to_i
     else
       raise ArgumentError.new("Expected dynamic value '*' to be an Int - #{value.inspect} (#{value.class.inspect})")
@@ -174,48 +178,44 @@ struct String::Formatter(A)
     {num, size}
   end
 
-  private def consume_type(flags, arg = nil, arg_specified = false)
+  private def consume_type(flags, index)
+    arg = arg_at(index)
+
     case char = current_char
     when 'c'
-      char flags, arg, arg_specified
+      char flags, arg
     when 's'
-      string flags, arg, arg_specified
+      string flags, arg
     when 'b'
       flags.base = 2
       flags.type = char
-      int flags, arg, arg_specified
+      int flags, arg
     when 'o'
       flags.base = 8
       flags.type = char
-      int flags, arg, arg_specified
+      int flags, arg
     when 'd', 'i'
       flags.base = 10
-      int flags, arg, arg_specified
+      int flags, arg
     when 'x', 'X'
       flags.base = 16
       flags.type = char
-      int flags, arg, arg_specified
+      int flags, arg
     when 'a', 'A', 'e', 'E', 'f', 'g', 'G'
       flags.type = char
-      float flags, arg, arg_specified
-    when '%'
-      char '%'
+      float flags, arg
     else
       raise ArgumentError.new("Malformed format string - %#{char.inspect}")
     end
   end
 
-  def char(flags, arg, arg_specified) : Nil
-    arg = next_arg unless arg_specified
-
+  def char(flags, arg) : Nil
     pad 1, flags if flags.left_padding?
     @io << arg
     pad 1, flags if flags.right_padding?
   end
 
-  def string(flags, arg, arg_specified) : Nil
-    arg = next_arg unless arg_specified
-
+  def string(flags, arg) : Nil
     if precision = flags.precision
       arg = arg.to_s[0...precision]
     end
@@ -225,9 +225,7 @@ struct String::Formatter(A)
     pad arg.to_s.size, flags if flags.right_padding?
   end
 
-  def int(flags, arg, arg_specified) : Nil
-    arg = next_arg unless arg_specified
-
+  def int(flags, arg) : Nil
     raise ArgumentError.new("Expected an integer, not #{arg.inspect}") unless arg.responds_to?(:to_i)
     int = arg.is_a?(Int) ? arg : arg.to_i
 
@@ -284,9 +282,7 @@ struct String::Formatter(A)
   end
 
   # We don't actually format the float ourselves, we delegate to snprintf
-  def float(flags, arg, arg_specified) : Nil
-    arg = next_arg unless arg_specified
-
+  def float(flags, arg) : Nil
     if arg.responds_to?(:to_f64)
       float = arg.is_a?(Float64) ? arg : arg.to_f64
 
@@ -367,16 +363,33 @@ struct String::Formatter(A)
     pad size, flags
   end
 
-  def char(char) : Nil
-    @io << char
+  private def arg_at(index : Nil)
+    args_are :sequential
+    arg = @args.fetch(@arg_index) { raise ArgumentError.new("Too few arguments") }
+    @arg_index += 1
+    arg
   end
 
-  private def current_arg
-    @args.fetch(@arg_index) { raise ArgumentError.new("Too few arguments") }
+  private def arg_at(index : String)
+    args_are :named
+    args = @args
+    # note: "index '0' out of bounds for empty tuple" without the `is_a?` check
+    # TODO: use `Tuple()` once support for 1.0.0 is dropped
+    if args.size == 1 && !args.is_a?(Tuple(*typeof(Tuple.new)))
+      arg = args[0]
+      if arg.is_a?(Hash) || arg.is_a?(NamedTuple)
+        return arg[index]
+      end
+    end
+    raise ArgumentError.new "One hash or named tuple required"
   end
 
-  def next_arg
-    current_arg.tap { @arg_index += 1 }
+  private def args_are(mode : Mode)
+    if @arg_mode.none?
+      @arg_mode = mode
+    elsif mode != @arg_mode
+      raise ArgumentError.new "Cannot mix #{@arg_mode.to_s.downcase} parameters with #{mode.to_s.downcase} ones"
+    end
   end
 
   private def current_char