diff --git a/src/ExCSS.Tests/SelectorsTests.cs b/src/ExCSS.Tests/SelectorsTests.cs
index 9c171f09..71013a56 100644
--- a/src/ExCSS.Tests/SelectorsTests.cs
+++ b/src/ExCSS.Tests/SelectorsTests.cs
@@ -89,6 +89,63 @@ public async Task FindAllStandardPseudoElementSelectors(bool allowInvalidSelecto
         Assert.Equal(expectedCount, list.Count());
     }
 
+    [Theory]
+    // "Whitespace is permitted on either side of the + or - that separates the An and B parts when both
+    // are present" (CSS Syntax 3 6.1), which lists "3n + 1" among its valid examples. The sign then
+    // arrives as a standalone delim token rather than as part of a signed <number>.
+    [InlineData(":nth-child(10n + 1)", 10, 1)]
+    [InlineData(":nth-child(10n - 1)", 10, -1)]
+    [InlineData(":nth-child(2n + 0)", 2, 0)]
+    [InlineData(":nth-child(-2n + 3)", -2, 3)]
+    [InlineData(":nth-child( 10n  +  1 )", 10, 1)]
+    // The compact form has always worked and must keep working.
+    [InlineData(":nth-child(10n+1)", 10, 1)]
+    [InlineData(":nth-child(10n-1)", 10, -1)]
+    [InlineData(":nth-child(odd)", 2, 1)]
+    [InlineData(":nth-child(even)", 2, 0)]
+    public void NthChildAcceptsAnBWithAndWithoutSpaces(string selector, int step, int offset)
+    {
+        var sheet = new StylesheetParser().Parse(selector + " { color: red }");
+
+        Assert.Equal(1, sheet.Rules.Length);
+        var child = Assert.IsAssignableFrom<ChildSelector>(((StyleRule)sheet.Rules[0]).Selector);
+        Assert.Equal(step, child.Step);
+        Assert.Equal(offset, child.Offset);
+    }
+
+    [Theory]
+    [InlineData(":nth-last-child(3n + 2)", 3, 2)]
+    [InlineData(":nth-of-type(3n + 2)", 3, 2)]
+    [InlineData(":nth-last-of-type(3n + 2)", 3, 2)]
+    public void NthVariantsAcceptSpacedSign(string selector, int step, int offset)
+    {
+        var sheet = new StylesheetParser().Parse(selector + " { color: red }");
+
+        Assert.Equal(1, sheet.Rules.Length);
+        var child = Assert.IsAssignableFrom<ChildSelector>(((StyleRule)sheet.Rules[0]).Selector);
+        Assert.Equal(step, child.Step);
+        Assert.Equal(offset, child.Offset);
+    }
+
+    [Theory]
+    // The production requires a <signless-integer> after a standalone sign, so a second sign is invalid.
+    // CSS Syntax 3 6.1 lists "3n + -6" among its invalid examples.
+    [InlineData(":nth-child(3n + -6)")]
+    [InlineData(":nth-child(10n + -1)")]
+    [InlineData(":nth-child(10n + +1)")]
+    [InlineData(":nth-child(10n + + 1)")]
+    [InlineData(":nth-child(10n + n)")]
+    // 6.1's invalid whitespace examples: the An part and the leading sign may not be split.
+    [InlineData(":nth-child(3 n)")]
+    [InlineData(":nth-child(+ 2n)")]
+    [InlineData(":nth-child(+ 2)")]
+    public void NthChildRejectsMalformedSpacedOffset(string selector)
+    {
+        var sheet = new StylesheetParser().Parse(selector + " { color: red }");
+
+        Assert.Equal(0, sheet.Rules.Length);
+    }
+
     private static bool HasStandardPseudoElementSelector(ISelector selector, bool negate = false)
     {
         if (selector is PseudoElementSelector pes)
diff --git a/src/ExCSS/Parser/SelectorConstructor.cs b/src/ExCSS/Parser/SelectorConstructor.cs
index c29678bf..bfcc3621 100644
--- a/src/ExCSS/Parser/SelectorConstructor.cs
+++ b/src/ExCSS/Parser/SelectorConstructor.cs
@@ -803,6 +803,7 @@ protected override bool OnToken(Token token)
                     ParseState.Initial => OnInitial(token),
                     ParseState.AfterInitialSign => OnAfterInitialSign(token),
                     ParseState.Offset => OnOffset(token),
+                    ParseState.AfterOffsetSign => OnAfterOffsetSign(token),
                     ParseState.BeforeOf => OnBeforeOf(token),
                     _ => OnAfter(token)
                 };
@@ -879,11 +880,39 @@ private bool OnOffset(Token token)
                         _offset *= _sign;
                         _state = ParseState.BeforeOf;
                         return false;
+                    case TokenType.Delim when token.Data.IsOneOf("+", "-"):
+                        // When whitespace separates the offset's sign from its digits the sign arrives as a
+                        // standalone delim token - the "<n-dimension> ['+' | '-'] <signless-integer>"
+                        // production of <a-n-plus-b> (CSS Syntax 3 6.2). The compact form "10n+1" instead
+                        // lexes as one signed <number> and is handled by the Number case above.
+                        _sign = token.Data == "-" ? -1 : +1;
+                        _state = ParseState.AfterOffsetSign;
+                        return false;
                     default:
                         return OnBeforeOf(token);
                 }
             }
 
+            private bool OnAfterOffsetSign(Token token)
+            {
+                switch (token.Type)
+                {
+                    case TokenType.Whitespace:
+                        return false;
+                    case TokenType.Number when !token.Data.StartsWith("+") && !token.Data.StartsWith("-"):
+                        // The production requires a <signless-integer> here, defined as "a <number-token>
+                        // with its type flag set to integer, and no sign character" (CSS Syntax 3 6.2), so
+                        // "10n + -1" and "10n + +1" are invalid - 6.1 lists "3n + -6" as an invalid example.
+                        _valid = _valid && ((NumberToken) token).IsInteger && int.TryParse(token.Data, out _offset);
+                        _offset *= _sign;
+                        _state = ParseState.BeforeOf;
+                        return false;
+                    default:
+                        _valid = false;
+                        return token.Type == TokenType.RoundBracketClose;
+                }
+            }
+
             private bool OnInitial(Token token)
             {
                 if (token.Type == TokenType.Whitespace) return false;
@@ -918,6 +947,7 @@ private enum ParseState : byte
                 Initial,
                 AfterInitialSign,
                 Offset,
+                AfterOffsetSign,
                 BeforeOf,
                 AfterOf
             }