diff --git a/README.rst b/README.rst
index 2c341dc..0d1fa9b 100644
--- a/README.rst
+++ b/README.rst
@@ -225,8 +225,8 @@ Mode-S Enhanced Surveillance (EHS)
 .. code:: python
 
   # BDS 4,0
-  pms.commb.alt40mcp(msg)   # MCP/FCU selected altitude (ft)
-  pms.commb.alt40fms(msg)   # FMS selected altitude (ft)
+  pms.commb.selalt40mcp(msg)   # MCP/FCU selected altitude (ft)
+  pms.commb.selalt40fms(msg)   # FMS selected altitude (ft)
   pms.commb.p40baro(msg)    # Barometric pressure (mb)
 
   # BDS 5,0
diff --git a/pyModeS/decoder/bds/bds40.py b/pyModeS/decoder/bds/bds40.py
index ef422e1..cf96cbe 100644
--- a/pyModeS/decoder/bds/bds40.py
+++ b/pyModeS/decoder/bds/bds40.py
@@ -18,10 +18,11 @@
 # Selected vertical intention
 # ------------------------------------------
 
-
 from __future__ import absolute_import, print_function, division
+import warnings
 from pyModeS.decoder.common import hex2bin, bin2int, data, allzeros, wrongstatus
 
+
 def is40(msg):
     """Check if a message is likely to be BDS code 4,0
 
@@ -65,7 +66,7 @@ def is40(msg):
     return True
 
 
-def alt40mcp(msg):
+def selalt40mcp(msg):
     """Selected altitude, MCP/FCU
 
     Args:
@@ -76,14 +77,14 @@ def alt40mcp(msg):
     """
     d = hex2bin(data(msg))
 
-    if d[0] == '0':
+    if d[0] == "0":
         return None
 
-    alt = bin2int(d[1:13]) * 16    # ft
+    alt = bin2int(d[1:13]) * 16  # ft
     return alt
 
 
-def alt40fms(msg):
+def selalt40fms(msg):
     """Selected altitude, FMS
 
     Args:
@@ -94,10 +95,10 @@ def alt40fms(msg):
     """
     d = hex2bin(data(msg))
 
-    if d[13] == '0':
+    if d[13] == "0":
         return None
 
-    alt = bin2int(d[14:26]) * 16    # ft
+    alt = bin2int(d[14:26]) * 16  # ft
     return alt
 
 
@@ -112,8 +113,26 @@ def p40baro(msg):
     """
     d = hex2bin(data(msg))
 
-    if d[26] == '0':
+    if d[26] == "0":
         return None
 
-    p = bin2int(d[27:39]) * 0.1 + 800    # millibar
+    p = bin2int(d[27:39]) * 0.1 + 800  # millibar
     return p
+
+
+def alt40mcp(msg):
+    warnings.simplefilter("once", DeprecationWarning)
+    warnings.warn(
+        "alt40mcp() has been renamed to selalt40mcp(). It will be removed in the future.",
+        DeprecationWarning,
+    )
+    return selalt40mcp(msg)
+
+
+def alt40fms(msg):
+    warnings.simplefilter("once", DeprecationWarning)
+    warnings.warn(
+        "alt40fms() has been renamed to selalt40fms(). It will be removed in the future.",
+        DeprecationWarning,
+    )
+    return selalt40mcp(msg)
diff --git a/tests/sample_run_commb.py b/tests/sample_run_commb.py
index 2ef3036..dde33ef 100644
--- a/tests/sample_run_commb.py
+++ b/tests/sample_run_commb.py
@@ -14,7 +14,7 @@ def bds_info(BDS, m):
         info = [commb.cs20(m)]
 
     elif BDS == "BDS40":
-        info = (commb.alt40mcp(m), commb.alt40fms(m), commb.p40baro(m))
+        info = (commb.selalt40mcp(m), commb.selalt40fms(m), commb.p40baro(m))
 
     elif BDS == "BDS44":
         info = (commb.wind44(m), commb.temp44(m), commb.p44(m), commb.hum44(m))
diff --git a/tests/test_commb.py b/tests/test_commb.py
index 9043765..6c0117e 100644
--- a/tests/test_commb.py
+++ b/tests/test_commb.py
@@ -10,12 +10,12 @@ def test_bds20_callsign():
 
 
 def test_bds40_functions():
-    assert bds.bds40.alt40mcp("A000029C85E42F313000007047D3") == 3008
-    assert bds.bds40.alt40fms("A000029C85E42F313000007047D3") == 3008
+    assert bds.bds40.selalt40mcp("A000029C85E42F313000007047D3") == 3008
+    assert bds.bds40.selalt40fms("A000029C85E42F313000007047D3") == 3008
     assert bds.bds40.p40baro("A000029C85E42F313000007047D3") == 1020.0
 
-    assert commb.alt40mcp("A000029C85E42F313000007047D3") == 3008
-    assert commb.alt40fms("A000029C85E42F313000007047D3") == 3008
+    assert commb.selalt40mcp("A000029C85E42F313000007047D3") == 3008
+    assert commb.selalt40fms("A000029C85E42F313000007047D3") == 3008
     assert commb.p40baro("A000029C85E42F313000007047D3") == 1020.0