diff --git a/draft-ietf-quic-transport.md b/draft-ietf-quic-transport.md
index 0c24a96669..3c1775a31b 100644
--- a/draft-ietf-quic-transport.md
+++ b/draft-ietf-quic-transport.md
@@ -4747,9 +4747,10 @@ QUIC acknowledgements are irrevocable.  Once acknowledged, a packet remains
 acknowledged, even if it does not appear in a future ACK frame.  This is unlike
 TCP SACKs ({{?RFC2018}}).
 
-An ACK frame acknowledges packets in the same packet number space as the packet
-in which it is contained.  As such, the same packet number can be acknowledged
-in different packet number spaces.
+The same packet number might be used to identify packets from multiple packet
+number spaces. An acknowledgement for a packet therefore needs to indicate both
+its packet number and its packet number space.  An ACK frame acknowledges packet
+numbers in the same space as the packet in which the ACK frame is contained.
 
 Version Negotiation and Retry packets cannot be acknowledged because they do not
 contain a packet number.  Rather than relying on ACK frames, these packets are