diff --git a/src/extended.rs b/src/extended.rs
index e69de29..670bfee 100644
--- a/src/extended.rs
+++ b/src/extended.rs
@@ -0,0 +1,121 @@
+use {
+    crate::{Error, PreRelease, MAX_U12},
+    serde::{Deserialize, Serialize},
+    std::{cmp::Ordering, fmt, str::FromStr},
+};
+
+#[derive(Clone, Copy, Debug, Deserialize, Serialize)]
+pub struct Extended {
+    pub major: u16,
+    pub minor: u16,
+    pub patch: u16,
+    pub build: u16,
+    pub pre: PreRelease,
+}
+
+impl fmt::Display for Extended {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "{}.{}.{}.{}",
+            self.major, self.minor, self.patch, self.build
+        )?;
+        match self.pre {
+            PreRelease::None => Ok(()),
+            p => write!(f, "_{p}"),
+        }
+    }
+}
+
+impl FromStr for Extended {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (s, pre) = match s.split_once('_') {
+            Some((a, b)) => (a, b.parse::<PreRelease>()?),
+            None => (s, PreRelease::None),
+        };
+        let split = s.split('.').collect::<Vec<_>>();
+        match split.len() {
+            3 => {
+                let major = split.first().unwrap().parse::<u16>()?;
+                let minor = split.get(1).unwrap().parse::<u16>()?;
+                let patch = split.get(2).unwrap().parse::<u16>()?;
+                let build = split.get(3).unwrap().parse::<u16>()?;
+                if major > MAX_U12 || minor > MAX_U12 || patch > MAX_U12 || build > MAX_U12 {
+                    return Err(Error::Range);
+                }
+                Ok(Self {
+                    major,
+                    minor,
+                    patch,
+                    build,
+                    pre,
+                })
+            }
+            _ => Err(Error::ParseSemver),
+        }
+    }
+}
+
+impl From<Extended> for u64 {
+    fn from(value: Extended) -> Self {
+        let major = u64::from(value.major) << 52;
+        let minor = u64::from(value.minor) << 40;
+        let patch = u64::from(value.patch) << 28;
+        let build = u64::from(value.build) << 14;
+        let pre = u64::from(u16::from(value.pre));
+        major | minor | patch | build | pre
+    }
+}
+
+impl TryFrom<u64> for Extended {
+    type Error = Error;
+
+    fn try_from(value: u64) -> Result<Self, Self::Error> {
+        let mut mask: u64 = 0o7777 << 52;
+        let major = (value & mask) >> 52;
+        mask = 0o7777 << 40;
+        let minor = (value & mask) >> 40;
+        mask = 0o7777 << 28;
+        let patch = (value & mask) >> 28;
+        mask = 0o7777 << 14;
+        let build = (value & mask) >> 14;
+        mask = 0o37777;
+        let p = u16::try_from(value & mask)?;
+        let pre: PreRelease = p.try_into()?;
+        Ok(Self {
+            major: major.try_into()?,
+            minor: minor.try_into()?,
+            patch: patch.try_into()?,
+            build: build.try_into()?,
+            pre,
+        })
+    }
+}
+
+impl PartialEq for Extended {
+    fn eq(&self, other: &Self) -> bool {
+        self.major == other.major
+            && self.minor == other.minor
+            && self.patch == other.patch
+            && self.build == other.build
+            && self.pre == other.pre
+    }
+}
+
+impl Eq for Extended {}
+
+impl Ord for Extended {
+    fn cmp(&self, other: &Self) -> Ordering {
+        let a = u64::from(*self);
+        let b = u64::from(*other);
+        a.cmp(&b)
+    }
+}
+
+impl PartialOrd for Extended {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 7069b1e..3558809 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,7 +1,8 @@
 mod error;
+mod extended;
 mod prerelease;
 mod semver;
 
-pub use {error::Error, prerelease::PreRelease, semver::SemVer};
+pub use {error::Error, extended::Extended, prerelease::PreRelease, semver::SemVer};
 
 pub static MAX_U12: u16 = 4096;
diff --git a/src/semver.rs b/src/semver.rs
index 576b1eb..9dfd282 100644
--- a/src/semver.rs
+++ b/src/semver.rs
@@ -144,7 +144,18 @@ mod tests {
 
     #[test]
     fn eq() {
-        todo!()
+        assert_eq!(
+            "1.0.0_alpha".parse::<SemVer>().unwrap(),
+            "1.0.0_alpha1".parse::<SemVer>().unwrap(),
+        );
+        assert_eq!(
+            "2.1.3_beta".parse::<SemVer>().unwrap(),
+            "2.1.3_beta1".parse::<SemVer>().unwrap(),
+        );
+        assert_eq!(
+            "1.11.0_rc".parse::<SemVer>().unwrap(),
+            "1.11.0_rc1".parse::<SemVer>().unwrap(),
+        );
     }
 
     #[test]