diff --git a/src/num.rs b/src/num.rs
index 25a0450..ff3aeab 100644
--- a/src/num.rs
+++ b/src/num.rs
@@ -284,6 +284,18 @@ impl_empty!(Signed, (i8, i16, i32, i64, i128, isize));
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_sign() {
+        let s = Sign::default();
+        assert_eq!(s, Sign::Positive);
+
+        let ms = -s;
+        assert_eq!(ms, Sign::Negative);
+
+        let ns = !s;
+        assert_eq!(ns, Sign::Negative);
+    }
+
     #[test]
     fn test_los() {
         assert_eq!(4u8.left_overflowing_sub(2).0, 2u8);
@@ -292,6 +304,8 @@ mod tests {
 
     #[test]
     fn test_gcd() {
+        assert_eq!(u8::gcd(5, 0), 5);
+        assert_eq!(u8::gcd(0, 5), 5);
         assert_eq!(u8::gcd(2, 3), 1);
         assert_eq!(u8::gcd(2, 2), 2);
         assert_eq!(u8::gcd(2, 8), 2);
diff --git a/src/rational.rs b/src/rational.rs
index f1ab36d..1083990 100644
--- a/src/rational.rs
+++ b/src/rational.rs
@@ -117,21 +117,21 @@ impl<T: Unsigned> Frac<T> {
 
     /// Determine the output sign given the two input signs
     fn get_sign(a: Self, b: Self, op: FracOp) -> Sign {
-        let mut output = Sign::default();
+        // -a + -b = -c
+        if op == FracOp::Addition && a.sign == Negative && b.sign == Negative {
+            return Negative;
+        }
 
-        if op == FracOp::Addition && !(a.sign == Positive && b.sign == Positive) {
-            output = Negative;
+        // a - -b = c
+        if op == FracOp::Subtraction && a.sign == Positive && b.sign == Negative {
+            return Positive;
         }
 
         if a.sign != b.sign {
-            output = if op == FracOp::Subtraction && b.sign == Negative {
-                Positive
-            } else {
-                Negative
-            }
+            Negative
+        } else {
+            Positive
         }
-
-        output
     }
 
     /// Convert the fraction to its simplest form
@@ -325,6 +325,28 @@ mod tests {
         Frac::raw(1u8, 0u8, Sign::default());
     }
 
+    #[test]
+    #[should_panic(expected = "Fraction can not have a zero denominator")]
+    fn zero_denom_new() {
+        frac!(1/0);
+    }
+
+    #[test]
+    fn test_get_sign() {
+        assert_eq!(Sign::Positive, Frac::get_sign(frac!(1), frac!(-1), FracOp::Subtraction));
+        assert_eq!(Sign::Negative, Frac::get_sign(frac!(-1), frac!(-1), FracOp::Addition));
+        assert_eq!(Sign::Negative, Frac::get_sign(frac!(-1), frac!(1), FracOp::Addition));
+        assert_eq!(Sign::Negative, Frac::get_sign(frac!(-1), frac!(1), FracOp::Subtraction));
+        assert_eq!(Sign::Negative, Frac::get_sign(frac!(-1), frac!(1), FracOp::Other));
+    }
+
+    #[test]
+    fn test_cmp() {
+        assert_eq!(Ordering::Greater, frac!(3/4).cmp(&frac!(1/4)));
+        assert_eq!(Ordering::Less, frac!(1/4).cmp(&frac!(3/4)));
+        assert_eq!(Ordering::Equal, frac!(1/2).cmp(&frac!(4/8)));
+    }
+
     #[test]
     fn macro_test() {
         let frac1 = frac!(1 / 3);