diff --git a/bip-0039.mediawiki b/bip-0039.mediawiki
index 2a6118b26a..82f99a4bec 100644
--- a/bip-0039.mediawiki
+++ b/bip-0039.mediawiki
@@ -96,6 +96,14 @@ sentence (in UTF-8 NFKD) used as the password and the string "mnemonic" + passph
 in UTF-8 NFKD) used as the salt. The iteration count is set to 2048 and HMAC-SHA512 is used as
 the pseudo-random function. The length of the derived key is 512 bits (= 64 bytes).
 
+Words in a mnemonic sentence are separated by a single space (U+0020), except in
+Japanese, where the ideographic space (U+3000) is used; see
+[[bip-0039/bip-0039-wordlists.md|Wordlists]]. NFKD maps U+3000 to U+0020, so the
+two forms of a Japanese sentence derive the same seed. Software that splits a
+sentence into words, to look them up in the wordlist or to verify the checksum,
+must split it after normalization: splitting on U+0020 beforehand reads a
+Japanese sentence as a single word.
+
 This seed can be later used to generate deterministic wallets using BIP-0032 or
 similar methods.