diff --git a/codespan/src/span.rs b/codespan/src/span.rs
index de80dcdf..a6c13106 100644
--- a/codespan/src/span.rs
+++ b/codespan/src/span.rs
@@ -44,11 +44,15 @@ impl Span {
         Span::new(0, s.len() as u32)
     }
 
-    /// Combine two spans by taking the start of the first span and the end of
-    /// the other span.
+    /// Combine two spans by taking the start of the earlier span
+    /// and the end of the later span.
+    ///
+    /// Note: this will work even if the two spans are disjoint.
+    /// If this doesn't make sense in your application, you should handle it yourself.
+    /// In that case, you can use `Span::disjoint` as a convenience function.
     ///
     /// ```rust
-    /// use codespan::{ByteIndex, Span};
+    /// use codespan::Span;
     ///
     /// let span1 = Span::new(0, 4);
     /// let span2 = Span::new(10, 16);
@@ -56,7 +60,28 @@ impl Span {
     /// assert_eq!(Span::merge(span1, span2), Span::new(0, 16));
     /// ```
     pub fn merge(self, other: Span) -> Span {
-        Span::new(self.start(), other.end())
+        use std::cmp::{max, min};
+
+        let start = min(self.start, other.start);
+        let end = max(self.end, other.end);
+        Span::new(start, end)
+    }
+
+    /// A helper function to tell whether two spans do not overlap.
+    ///
+    /// ```
+    /// use codespan::Span;
+    /// let span1 = Span::new(0, 4);
+    /// let span2 = Span::new(10, 16);
+    /// assert!(span1.disjoint(span2));
+    /// ```
+    pub fn disjoint(self, other: Span) -> bool {
+        let (first, last) = if self.end < other.end {
+            (self, other)
+        } else {
+            (other, self)
+        };
+        first.end <= last.start
     }
 
     /// Get the starting byte index.
@@ -111,3 +136,63 @@ where
         Span::new(range.start, range.end)
     }
 }
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn test_merge() {
+        use super::Span;
+
+        // overlap
+        let a = Span::from(1..5);
+        let b = Span::from(3..10);
+        assert_eq!(a.merge(b), Span::from(1..10));
+        assert_eq!(b.merge(a), Span::from(1..10));
+
+        // subset
+        let two_four = (2..4).into();
+        assert_eq!(a.merge(two_four), (1..5).into());
+        assert_eq!(two_four.merge(a), (1..5).into());
+
+        // disjoint
+        let ten_twenty = (10..20).into();
+        assert_eq!(a.merge(ten_twenty), (1..20).into());
+        assert_eq!(ten_twenty.merge(a), (1..20).into());
+
+        // identity
+        assert_eq!(a.merge(a), a);
+    }
+
+    #[test]
+    fn test_disjoint() {
+        use super::Span;
+
+        // overlap
+        let a = Span::from(1..5);
+        let b = Span::from(3..10);
+        assert!(!a.disjoint(b));
+        assert!(!b.disjoint(a));
+
+        // subset
+        let two_four = (2..4).into();
+        assert!(!a.disjoint(two_four));
+        assert!(!two_four.disjoint(a));
+
+        // disjoint
+        let ten_twenty = (10..20).into();
+        assert!(a.disjoint(ten_twenty));
+        assert!(ten_twenty.disjoint(a));
+
+        // identity
+        assert!(!a.disjoint(a));
+
+        // off by one (upper bound)
+        let c = Span::from(5..10);
+        assert!(a.disjoint(c));
+        assert!(c.disjoint(a));
+        // off by one (lower bound)
+        let d = Span::from(0..1);
+        assert!(a.disjoint(d));
+        assert!(d.disjoint(a));
+    }
+}