diff --git a/src/greaseweazle/codec/codec.py b/src/greaseweazle/codec/codec.py
index fe4934a5..5e2f6f00 100644
--- a/src/greaseweazle/codec/codec.py
+++ b/src/greaseweazle/codec/codec.py
@@ -5,6 +5,7 @@
 # This is free and unencumbered software released into the public domain.
 # See the file COPYING for more details, or visit <http://unlicense.org>.
 
+from __future__ import annotations
 from typing import Dict, List, Tuple, Optional
 
 import os.path, re
@@ -128,16 +129,37 @@ def default_revs(self) -> float:
         return max([x.default_revs for x in self.track_map.values()])
 
 
-
-def read_diskdef_file_lines(filename: Optional[str]) -> Tuple[List[str], str]:
-    if filename is None:
-        filename = 'diskdefs.cfg'
-        with importlib.resources.open_text('greaseweazle.data', filename) as f:
-            lines = f.readlines()
-    else:
-        with open(os.path.expanduser(filename), 'r') as f:
-            lines = f.readlines()
-    return (lines, filename)
+class DiskDef_File:
+    def __init__(self, name: Optional[str],
+                 parent: Optional[DiskDef_File] = None,
+                 prefix: Optional[str] = None) -> None:
+        self.linenr = 0
+        self.parent = parent
+        self.prefix = prefix
+        self.path: Optional[str] = None
+        self.name: str = 'diskdefs.cfg' if name is None else name
+        if name is None or (self.parent and not self.parent.path):
+            with importlib.resources.open_text('greaseweazle.data',
+                                               self.name) as f:
+                self.lines = f.readlines()
+        else:
+            if self.parent:
+                assert self.parent.path # mypy
+                self.path = os.path.join(os.path.dirname(self.parent.path),
+                                         self.name)
+            else:
+                self.path = os.path.expanduser(self.name)
+            with open(self.path, 'r') as f:
+                self.lines = f.readlines()
+
+    def full_prefix(self) -> str:
+        s: List[str] = []
+        p: Optional[DiskDef_File] = self
+        while p is not None:
+            if p.prefix:
+                s.insert(0, p.prefix)
+            p = p.parent
+        return '.'.join(s) + '.' if s else ''
 
 
 # Import the TrackDef subclasses
@@ -177,18 +199,18 @@ class ParseMode:
     Disk  = 1
     Track = 2
 
-def get_diskdef(
+def _get_diskdef(
         format_name: str,
-        diskdef_filename: Optional[str] = None
+        diskdef_file: DiskDef_File
 ) -> Optional[DiskDef]:
 
     parse_mode = ParseMode.Outer
     active = False
+    prefix = diskdef_file.full_prefix()
     disk: Optional[DiskDef] = None
     track: Optional[TrackDef] = None
-    lines, diskdef_filename = read_diskdef_file_lines(diskdef_filename)
 
-    for linenr, l in enumerate(lines, start=1):
+    for diskdef_file.linenr, l in enumerate(diskdef_file.lines, start=1):
         try:
             # Strip comments and whitespace.
             match = re.match(r'\s*([^#]*)', l)
@@ -201,17 +223,28 @@ def get_diskdef(
 
             if parse_mode == ParseMode.Outer:
                 disk_match = re.match(r'disk\s+([\w,.-]+)', t)
-                error.check(disk_match is not None, 'syntax error')
-                assert disk_match is not None # mypy
-                parse_mode = ParseMode.Disk
-                active = disk_match.group(1) == format_name
-                if active:
-                    disk = DiskDef()
+                if disk_match:
+                    parse_mode = ParseMode.Disk
+                    active = prefix + disk_match.group(1) == format_name
+                    if active:
+                        disk = DiskDef()
+                else:
+                    prefix_match = re.match(r'prefix\s+([\w,.-]+)\s+(\S+)', t)
+                    error.check(prefix_match is not None, 'syntax error')
+                    assert prefix_match is not None # mypy
+                    disk = _get_diskdef(format_name, DiskDef_File(
+                        name = prefix_match.group(2),
+                        prefix = prefix_match.group(1),
+                        parent = diskdef_file))
+                    if disk:
+                        break
 
             elif parse_mode == ParseMode.Disk:
                 if t == 'end':
                     parse_mode = ParseMode.Outer
                     active = False
+                    if disk:
+                        break
                     continue
                 tracks_match = re.match(r'tracks\s+([0-9,.*-]+)'
                                         '\s+([\w,.-]+)', t)
@@ -287,24 +320,44 @@ def get_diskdef(
                                 keyval_match.group(2))
 
         except Exception as err:
-            ctxt = "%s, line %d: " % (diskdef_filename, linenr)
-            err.args = (ctxt + err.args[0],) + err.args[1:]
+            if err.args and isinstance(x := err.args[0], str):
+                ctxt = f'At {diskdef_file.name}, line {diskdef_file.linenr}:'
+                ctxt += '\n' if x.startswith('At') else ' '
+                err.args = (ctxt + x,) + err.args[1:]
             raise
 
+    return disk
+
+def get_diskdef(
+        format_name: str,
+        diskdef_filename: Optional[str] = None
+) -> Optional[DiskDef]:
+    diskdef_file = DiskDef_File(name = diskdef_filename)
+    disk = _get_diskdef(format_name, diskdef_file)
     if disk is None:
         return None
     disk.finalise()
-
     return disk
 
-
-def print_formats(diskdef_filename: Optional[str] = None) -> str:
-    columns, sep, formats = 80, 2, []
-    lines, _ = read_diskdef_file_lines(diskdef_filename)
-    for l in lines:
+def get_all_formats(diskdef_file: DiskDef_File) -> List[str]:
+    formats = []
+    prefix = diskdef_file.full_prefix()
+    for diskdef_file.linenr, l in enumerate(diskdef_file.lines, start=1):
         disk_match = re.match(r'\s*disk\s+([\w,.-]+)', l)
         if disk_match:
-            formats.append(disk_match.group(1))
+            formats.append(prefix + disk_match.group(1))
+        prefix_match = re.match(r'\s*prefix\s+([\w,.-]+)\s+(\S+)', l)
+        if prefix_match:
+            formats += get_all_formats(DiskDef_File(
+                name = prefix_match.group(2),
+                prefix = prefix_match.group(1),
+                parent = diskdef_file))
+    return formats
+
+def print_formats(diskdef_filename: Optional[str] = None) -> str:
+    columns, sep = 80, 2
+    diskdef_file = DiskDef_File(name = diskdef_filename)
+    formats = get_all_formats(diskdef_file)
     formats.sort()
     return util.columnify(formats)
 
diff --git a/src/greaseweazle/data/diskdefs.cfg b/src/greaseweazle/data/diskdefs.cfg
index 20c274f1..7cd4eb7d 100644
--- a/src/greaseweazle/data/diskdefs.cfg
+++ b/src/greaseweazle/data/diskdefs.cfg
@@ -794,144 +794,7 @@ disk hp.mmfm.9895
     end
 end
 
-disk ibm.160
-    cyls = 40
-    heads = 1
-    tracks * ibm.mfm
-        secs = 8
-        bps = 512
-        gap3 = 84
-        rate = 250
-    end
-end
-
-disk ibm.180
-    cyls = 40
-    heads = 1
-    tracks * ibm.mfm
-        secs = 9
-        bps = 512
-        gap3 = 84
-        rate = 250
-    end
-end
-
-disk ibm.320
-    cyls = 40
-    heads = 2
-    tracks * ibm.mfm
-        secs = 8
-        bps = 512
-        gap3 = 84
-        rate = 250
-    end
-end
-
-disk ibm.360
-    cyls = 40
-    heads = 2
-    tracks * ibm.mfm
-        secs = 9
-        bps = 512
-        gap3 = 84
-        rate = 250
-    end
-end
-
-disk ibm.720
-    cyls = 80
-    heads = 2
-    tracks * ibm.mfm
-        secs = 9
-        bps = 512
-        gap3 = 84
-        rate = 250
-    end
-end
-
-disk ibm.800
-    cyls = 80
-    heads = 2
-    tracks * ibm.mfm
-        secs = 10
-        bps = 512
-        gap3 = 30
-        rate = 250
-    end
-end
-
-disk ibm.1200
-    cyls = 80
-    heads = 2
-    tracks * ibm.mfm
-        secs = 15
-        bps = 512
-        gap3 = 84
-        rate = 500
-        rpm = 360
-    end
-end
-
-disk ibm.1440
-    cyls = 80
-    heads = 2
-    tracks * ibm.mfm
-        secs = 18
-        bps = 512
-        gap3 = 84
-        rate = 500
-    end
-end
-
-disk ibm.1680
-    cyls = 80
-    heads = 2
-    tracks * ibm.mfm
-        secs = 21
-        bps = 512
-        gap3 = 12
-        cskew = 3
-        interleave = 2
-        rate = 500
-    end
-end
-
-# alias for ibm.1680
-disk ibm.dmf
-    cyls = 80
-    heads = 2
-    tracks * ibm.mfm
-        secs = 21
-        bps = 512
-        gap3 = 12
-        cskew = 3
-        interleave = 2
-        rate = 500
-    end
-end
-
-disk ibm.2880
-    cyls = 80
-    heads = 2
-    tracks * ibm.mfm
-        secs = 36
-        bps = 512
-        gap2 = 41
-        rate = 1000
-    end
-end
-
-# Generic IBM format codec which will scan a track for any valid FM/MFM
-# sectors at standard rates (125kbps, 250kbps, 500kbps) and RPMs (300, 360).
-disk ibm.scan
-    cyls = 80
-    heads = 2
-    tracks * ibm.scan
-        # Following options restrict the scanner's search
-        # rate = 250
-        # rpm = 300
-    end
-end
+prefix ibm diskdefs_ibm.cfg
 
 disk mac.400
     cyls = 80
diff --git a/src/greaseweazle/data/diskdefs_ibm.cfg b/src/greaseweazle/data/diskdefs_ibm.cfg
new file mode 100644
index 00000000..41174680
--- /dev/null
+++ b/src/greaseweazle/data/diskdefs_ibm.cfg
@@ -0,0 +1,138 @@
+disk 160
+    cyls = 40
+    heads = 1
+    tracks * ibm.mfm
+        secs = 8
+        bps = 512
+        gap3 = 84
+        rate = 250
+    end
+end
+
+disk 180
+    cyls = 40
+    heads = 1
+    tracks * ibm.mfm
+        secs = 9
+        bps = 512
+        gap3 = 84
+        rate = 250
+    end
+end
+
+disk 320
+    cyls = 40
+    heads = 2
+    tracks * ibm.mfm
+        secs = 8
+        bps = 512
+        gap3 = 84
+        rate = 250
+    end
+end
+
+disk 360
+    cyls = 40
+    heads = 2
+    tracks * ibm.mfm
+        secs = 9
+        bps = 512
+        gap3 = 84
+        rate = 250
+    end
+end
+
+disk 720
+    cyls = 80
+    heads = 2
+    tracks * ibm.mfm
+        secs = 9
+        bps = 512
+        gap3 = 84
+        rate = 250
+    end
+end
+
+disk 800
+    cyls = 80
+    heads = 2
+    tracks * ibm.mfm
+        secs = 10
+        bps = 512
+        gap3 = 30
+        rate = 250
+    end
+end
+
+disk 1200
+    cyls = 80
+    heads = 2
+    tracks * ibm.mfm
+        secs = 15
+        bps = 512
+        gap3 = 84
+        rate = 500
+        rpm = 360
+    end
+end
+
+disk 1440
+    cyls = 80
+    heads = 2
+    tracks * ibm.mfm
+        secs = 18
+        bps = 512
+        gap3 = 84
+        rate = 500
+    end
+end
+
+disk 1680
+    cyls = 80
+    heads = 2
+    tracks * ibm.mfm
+        secs = 21
+        bps = 512
+        gap3 = 12
+        cskew = 3
+        interleave = 2
+        rate = 500
+    end
+end
+
+# alias for 1680
+disk dmf
+    cyls = 80
+    heads = 2
+    tracks * ibm.mfm
+        secs = 21
+        bps = 512
+        gap3 = 12
+        cskew = 3
+        interleave = 2
+        rate = 500
+    end
+end
+
+disk 2880
+    cyls = 80
+    heads = 2
+    tracks * ibm.mfm
+        secs = 36
+        bps = 512
+        gap2 = 41
+        rate = 1000
+    end
+end
+
+# Generic IBM format codec which will scan a track for any valid FM/MFM
+# sectors at standard rates (125kbps, 250kbps, 500kbps) and RPMs (300, 360).
+disk scan
+    cyls = 80
+    heads = 2
+    tracks * ibm.scan
+        # Following options restrict the scanner's search
+        # rate = 250
+        # rpm = 300
+    end
+end