diff --git a/hledger-lib/Hledger/Data/Journal.hs b/hledger-lib/Hledger/Data/Journal.hs
index ef50d6571..8477d5cee 100644
--- a/hledger-lib/Hledger/Data/Journal.hs
+++ b/hledger-lib/Hledger/Data/Journal.hs
@@ -737,18 +737,19 @@ journalApplyCommodityStyles j@Journal{jtxns=ts, jmarketprices=mps} = j''
       fixposting p@Posting{pamount=a} = p{pamount=styleMixedAmount styles a}
       fixmarketprice mp@MarketPrice{mpamount=a} = mp{mpamount=styleAmount styles a}
 
--- | Get all the amount styles defined in this journal, either 
--- declared by a commodity directive (preferred) or inferred from amounts,
--- as a map from symbol to style. 
+-- | Get all the amount styles defined in this journal, either declared by 
+-- a commodity directive or inferred from amounts, as a map from symbol to style. 
+-- Styles declared by commodity directives take precedence, and these also are
+-- guaranteed to know their decimal point character.
 journalCommodityStyles :: Journal -> M.Map CommoditySymbol AmountStyle
 journalCommodityStyles j = declaredstyles <> inferredstyles
   where
     declaredstyles = M.mapMaybe cformat $ jcommodities j
     inferredstyles = jinferredcommodities j
 
--- | Infer a display format for each commodity based on the amounts parsed.
--- "hledger... will use the format of the first posting amount in the
--- commodity, and the highest precision of all posting amounts in the commodity."
+-- | Collect and save inferred amount styles for each commodity based on
+-- the posting amounts in that commodity (excluding price amounts), ie:
+-- "the format of the first amount, adjusted to the highest precision of all amounts".
 journalInferCommodityStyles :: Journal -> Journal
 journalInferCommodityStyles j =
   j{jinferredcommodities =
diff --git a/hledger-lib/Hledger/Read/JournalReader.hs b/hledger-lib/Hledger/Read/JournalReader.hs
index 58641a836..a42d7be81 100644
--- a/hledger-lib/Hledger/Read/JournalReader.hs
+++ b/hledger-lib/Hledger/Read/JournalReader.hs
@@ -274,15 +274,21 @@ commoditydirectivep = try commoditydirectiveonelinep <|> commoditydirectivemulti
 --
 -- >>> Right _ <- rejp commoditydirectiveonelinep "commodity $1.00"
 -- >>> Right _ <- rejp commoditydirectiveonelinep "commodity $1.00 ; blah\n"
-commoditydirectiveonelinep :: Monad m => JournalParser m ()
+commoditydirectiveonelinep :: Monad m => ErroringJournalParser m ()
 commoditydirectiveonelinep = do
   string "commodity"
   lift (skipSome spacenonewline)
+  pos <- getPosition
   Amount{acommodity,astyle} <- amountp
   lift (skipMany spacenonewline)
   _ <- followingcommentp <|> (lift eolof >> return "")
-  let comm = Commodity{csymbol=acommodity, cformat=Just astyle}
-  modify' (\j -> j{jcommodities=M.insert acommodity comm $ jcommodities j})
+  let comm = Commodity{csymbol=acommodity, cformat=Just $ dbg2 "style from commodity directive" astyle}
+  if asdecimalpoint astyle == Nothing
+  then parserErrorAt pos pleaseincludedecimalpoint
+  else modify' (\j -> j{jcommodities=M.insert acommodity comm $ jcommodities j})
+
+pleaseincludedecimalpoint :: String
+pleaseincludedecimalpoint = "to avoid ambiguity, please include a decimal point in commodity directives"
 
 -- | Parse a multi-line commodity directive, containing 0 or more format subdirectives.
 --
@@ -309,7 +315,10 @@ formatdirectivep expectedsym = do
   Amount{acommodity,astyle} <- amountp
   _ <- followingcommentp <|> (lift eolof >> return "")
   if acommodity==expectedsym
-    then return astyle
+    then 
+      if asdecimalpoint astyle == Nothing
+      then parserErrorAt pos pleaseincludedecimalpoint
+      else return $ dbg2 "style from format subdirective" astyle
     else parserErrorAt pos $
          printf "commodity directive symbol \"%s\" and format directive symbol \"%s\" should be the same" expectedsym acommodity
 
@@ -395,13 +404,16 @@ defaultyeardirectivep = do
   failIfInvalidYear y
   setYear y'
 
-defaultcommoditydirectivep :: Monad m => JournalParser m ()
+defaultcommoditydirectivep :: Monad m => ErroringJournalParser m ()
 defaultcommoditydirectivep = do
   char 'D' <?> "default commodity"
   lift (skipSome spacenonewline)
-  Amount{..} <- amountp
+  pos <- getPosition
+  Amount{acommodity,astyle} <- amountp
   lift restofline
-  setDefaultCommodityAndStyle (acommodity, astyle)
+  if asdecimalpoint astyle == Nothing
+  then parserErrorAt pos pleaseincludedecimalpoint
+  else setDefaultCommodityAndStyle (acommodity, astyle)
 
 marketpricedirectivep :: Monad m => JournalParser m MarketPrice
 marketpricedirectivep = do
diff --git a/hledger-lib/hledger_journal.m4.md b/hledger-lib/hledger_journal.m4.md
index 526b33969..ab239c41f 100644
--- a/hledger-lib/hledger_journal.m4.md
+++ b/hledger-lib/hledger_journal.m4.md
@@ -799,9 +799,7 @@ line containing just `end comment` ends it. See [comments](#comments).
 
 ### commodity directive
 
-The `commodity` directive predefines commodities (currently this is just informational),
-and also it may define the display format for amounts in this commodity (overriding the automatically inferred format).
-
+The `commodity` directive declares commodities which may be used in the journal (though currently we do not enforce this).
 It may be written on a single line, like this:
 
 ```journal
@@ -827,6 +825,12 @@ commodity INR
   format INR 9,99,99,999.00
 ```
 
+Commodity directives have a second purpose: they define the standard display format for amounts in the commodity. 
+Normally the display format is inferred from journal entries, but this can be unpredictable; 
+declaring it with a commodity directive overrides this and removes ambiguity. 
+Towards this end, amounts in commodity directives must always be written with a decimal point 
+(a period or comma, followed by 0 or more decimal digits). 
+
 ### Default commodity
 
 The D directive sets a default commodity (and display format), to be used for amounts without a commodity symbol (ie, plain numbers).
@@ -843,6 +847,8 @@ D $1,000.00
   b
 ```
 
+As with the `commodity` directive, the amount must always be written with a decimal point. 
+
 ### Default year
 
 You can set a default year to be used for subsequent dates which don't
diff --git a/tests/journal/numbers.test b/tests/journal/numbers.test
index 05ff85ef5..ba2cf51b7 100644
--- a/tests/journal/numbers.test
+++ b/tests/journal/numbers.test
@@ -126,26 +126,17 @@ commodity €1,000.00
 >>>2
 >>>=0
 
-# 11. No decimals but have hint from commodity directive with groups
+# 11. Commodity directive requires a decimal point
 hledger bal -f -
 <<<
-commodity 1,000,000 EUR
+commodity 1000 EUR
+>>>2 /please include a decimal point/
+>>>=1
 
-2017/1/1
-	a   1,000 EUR
-	b  -1,000.00 EUR
->>>
-           1,000 EUR  a
-          -1,000 EUR  b
---------------------
-                   0
->>>2
->>>=0
-
-# 12. No decimals but have hint from commodity directive with zero precision
+# 12. Commodity directive with zero precision
 hledger bal -f -
 <<<
-commodity 100 EUR
+commodity 100. EUR
 
 2017/1/1
 	a   1,000 EUR