diff --git a/src/lib.rs b/src/lib.rs
index 65fcf13..be74d38 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -54,6 +54,9 @@
 //!   images, which were distinguished by `bool` previously.  Function
 //!   parameters and struct members now take `In`s instead of `bool`s.
 //!   `Field::thumbnail` was renamed to `Field::ifd_num` accordingly.
+//! * The type of `Context` was changed from enum to struct.  The variants
+//!   (e.g., `Context::Tiff`) were changed to associated constants and
+//!   they are now spelled in all uppercase (e.g., `Context::TIFF`).
 
 pub use error::Error;
 pub use jpeg::get_exif_attr as get_exif_attr_from_jpeg;
diff --git a/src/tag.rs b/src/tag.rs
index ce2fde0..139dc99 100644
--- a/src/tag.rs
+++ b/src/tag.rs
@@ -60,8 +60,8 @@ impl Tag {
     /// # Examples
     /// ```
     /// use exif::{Context, Tag};
-    /// assert_eq!(Tag::DateTime.context(), Context::Tiff);
-    /// assert_eq!(Tag::ExposureTime.context(), Context::Exif);
+    /// assert_eq!(Tag::DateTime.context(), Context::TIFF);
+    /// assert_eq!(Tag::ExposureTime.context(), Context::EXIF);
     /// ```
     #[inline]
     pub fn context(self) -> Context {
@@ -102,22 +102,75 @@ impl fmt::Display for Tag {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match get_tag_info(*self) {
             Some(ti) => f.pad(ti.name),
-            None => f.pad(&format!("{:?}", self)),
+            None => f.pad(&format!("Tag({}, {})", self.0, self.1)),
         }
     }
 }
 
-/// An enum that indicates how a tag number is interpreted.
+/// A type that indicates how a tag number is interpreted.
+// This type is not an enum to keep API compatibilities when
+// new contexts are introduced.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
-pub enum Context {
+pub struct Context(std::num::NonZeroU16);
+
+macro_rules! generate_context {
+    (
+        $(
+            // Copy the doc attribute to the actual definition.
+            $( #[$attr:meta] )*
+            $name: ident = $num: expr, $disp: expr;
+        )+
+    ) => (
+        impl Context {
+            $(
+                $( #[$attr] )*
+                pub const $name: Self = Self(
+                    unsafe { std::num::NonZeroU16::new_unchecked($num) });
+            )+
+        }
+
+        fn display_context(ctx: &Context, f: &mut fmt::Formatter)
+                           -> fmt::Result {
+            let name = match ctx.0.get() {
+                $( $num => $disp, )+
+                _ => return fmt::Debug::fmt(ctx, f),
+            };
+            f.write_str(name)
+        }
+    )
+}
+
+generate_context! {
     /// TIFF attributes defined in the TIFF Rev. 6.0 specification.
-    Tiff,	// 0th/1st IFD
+    TIFF = 1, "TIFF";		// 0th/1st IFD (toplevel)
     /// Exif attributes.
-    Exif,	// 0th/1st IFD -- Exif IFD
+    EXIF = 2, "Exif";		// -- Exif IFD
     /// GPS attributes.
-    Gps,	// 0th/1st IFD -- GPS IFD
+    GPS = 3, "GPS";		// -- GPS IFD
     /// Interoperability attributes.
-    Interop,	// 0th/1st IFD -- Exif IFD -- Interoperability IFD
+    INTEROP = 4, "Interop";	// -- Exif IFD -- Interoperability IFD
+}
+
+impl Context {
+    // Compatibility with 0.3.x.
+    #[allow(non_upper_case_globals)]
+    #[deprecated(since = "0.4.0", note = "use Context::TIFF instead")]
+    pub const Tiff: Self = Self::TIFF;
+    #[allow(non_upper_case_globals)]
+    #[deprecated(since = "0.4.0", note = "use Context::EXIF instead")]
+    pub const Exif: Self = Self::EXIF;
+    #[allow(non_upper_case_globals)]
+    #[deprecated(since = "0.4.0", note = "use Context::GPS instead")]
+    pub const Gps: Self = Self::GPS;
+    #[allow(non_upper_case_globals)]
+    #[deprecated(since = "0.4.0", note = "use Context::INTEROP instead")]
+    pub const Interop: Self = Self::INTEROP;
+}
+
+impl fmt::Display for Context {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        display_context(self, f)
+    }
 }
 
 #[derive(Debug)]
@@ -215,7 +268,7 @@ macro_rules! generate_well_known_tag_constants {
 // the Exif field names: camel cases and all-capital acronyms.
 generate_well_known_tag_constants!(
     // Exif-specific IFDs [EXIF23 4.6.3].
-    |Context::Tiff|
+    |Context::TIFF|
 
     /// A pointer to the Exif IFD.  This is used for the internal structure
     /// of Exif data and will not be returned to the user.
@@ -228,7 +281,7 @@ generate_well_known_tag_constants!(
      unit![],
      "GPS Info IFD pointer"),
 
-    |Context::Exif|
+    |Context::EXIF|
 
     /// A pointer to the interoperability IFD.  This is used for the internal
     /// structure of Exif data and will not be returned to the user.
@@ -238,7 +291,7 @@ generate_well_known_tag_constants!(
 
     // TIFF primary and thumbnail attributes [EXIF23 4.6.4 Table 4,
     // 4.6.8 Table 17, and 4.6.8 Table 21].
-    |Context::Tiff|
+    |Context::TIFF|
 
     (ImageWidth, 0x100, DefaultValue::None, d_default,
      unit!["pixels"],
@@ -340,7 +393,7 @@ generate_well_known_tag_constants!(
      "Copyright holder"),
 
     // Exif IFD attributes [EXIF23 4.6.5 Table 7 and 4.6.8 Table 18].
-    |Context::Exif|
+    |Context::EXIF|
 
     (ExposureTime, 0x829a, DefaultValue::None, d_exptime,
      unit!["s"],
@@ -582,7 +635,7 @@ generate_well_known_tag_constants!(
      "Gamma"),
 
     // GPS attributes [EXIF23 4.6.6 Table 15 and 4.6.8 Table 19].
-    |Context::Gps|
+    |Context::GPS|
 
     // Depends on the Exif version.
     (GPSVersionID, 0x0, DefaultValue::ContextDependent, d_gpsver,
@@ -683,7 +736,7 @@ generate_well_known_tag_constants!(
      "Horizontal positioning error"),
 
     // Interoperability attributes [EXIF23 4.6.7 Table 16 and 4.6.8 Table 20].
-    |Context::Interop|
+    |Context::INTEROP|
 
     (InteroperabilityIndex, 0x1, DefaultValue::None, d_default,
      unit![],
@@ -1456,12 +1509,12 @@ mod tests {
     #[test]
     fn tag_constant_in_pattern() {
         // Destructuring, which will always work.
-        match Tag(Context::Tiff, 0x132) {
-            Tag(Context::Tiff, 0x132) => {},
+        match Tag(Context::TIFF, 0x132) {
+            Tag(Context::TIFF, 0x132) => {},
             _ => panic!("failed to match Tag"),
         }
         // Matching against a constant.  Test if this compiles.
-        match Tag(Context::Tiff, 0x132) {
+        match Tag(Context::TIFF, 0x132) {
             Tag::DateTime => {},
             _ => panic!("failed to match Tag"),
         }
@@ -1498,11 +1551,11 @@ mod tests {
 
     #[test]
     fn tag_fmt_display() {
-        let tag1 = Tag(Context::Tiff, 0x132);
+        let tag1 = Tag(Context::TIFF, 0x132);
         assert_eq!(format!("{:15}", tag1), "DateTime       ");
         assert_eq!(format!("{:>15}", tag1), "       DateTime");
         assert_eq!(format!("{:5.6}", tag1), "DateTi");
-        let tag2 = Tag(Context::Exif, 0);
+        let tag2 = Tag(Context::EXIF, 0);
         assert_eq!(format!("{:15}", tag2), "Tag(Exif, 0)   ");
         assert_eq!(format!("{:>15}", tag2), "   Tag(Exif, 0)");
         assert_eq!(format!("{:5.6}", tag2), "Tag(Ex");
diff --git a/src/tiff.rs b/src/tiff.rs
index e8bd99a..0112596 100644
--- a/src/tiff.rs
+++ b/src/tiff.rs
@@ -122,7 +122,7 @@ fn parse_exif_sub<E>(data: &[u8])
             return Err(Error::InvalidFormat("Limit the IFD count to 8"));
         }
         ifd_offset = parse_ifd::<E>(
-            &mut fields, data, ifd_offset, Context::Tiff, ifd_num)?;
+            &mut fields, data, ifd_offset, Context::TIFF, ifd_num)?;
         ifd_num_ck = ifd_num.checked_add(1);
     }
     Ok(fields)
@@ -168,11 +168,11 @@ fn parse_ifd<'a, E>(fields: &mut Vec<Field<'a>>, data: &'a [u8],
         let tag = Tag(ctx, tag);
         match tag {
             Tag::ExifIFDPointer => parse_child_ifd::<E>(
-                fields, data, &val, Context::Exif, ifd_num)?,
+                fields, data, &val, Context::EXIF, ifd_num)?,
             Tag::GPSInfoIFDPointer => parse_child_ifd::<E>(
-                fields, data, &val, Context::Gps, ifd_num)?,
+                fields, data, &val, Context::GPS, ifd_num)?,
             Tag::InteropIFDPointer => parse_child_ifd::<E>(
-                fields, data, &val, Context::Interop, ifd_num)?,
+                fields, data, &val, Context::INTEROP, ifd_num)?,
             _ => fields.push(Field {
                 tag: tag, ifd_num: In(ifd_num), value: val }),
         }
diff --git a/src/writer.rs b/src/writer.rs
index 73f377e..f48f630 100644
--- a/src/writer.rs
+++ b/src/writer.rs
@@ -130,10 +130,11 @@ impl<'a> Writer<'a> {
             Field { tag: Tag(ctx, _), ifd_num, .. } => {
                 let ifd = self.pick_ifd(ifd_num);
                 match ctx {
-                    Context::Tiff => ifd.tiff_fields.push(field),
-                    Context::Exif => ifd.exif_fields.push(field),
-                    Context::Gps => ifd.gps_fields.push(field),
-                    Context::Interop => ifd.interop_fields.push(field),
+                    Context::TIFF => ifd.tiff_fields.push(field),
+                    Context::EXIF => ifd.exif_fields.push(field),
+                    Context::GPS => ifd.gps_fields.push(field),
+                    Context::INTEROP => ifd.interop_fields.push(field),
+                    _ => unimplemented!(),
                 }
             },
         }