diff --git a/net-mail/notmuch/notmuch-0.18.2.ebuild b/net-mail/notmuch/notmuch-0.18.2.ebuild
index f8d0eb4b2e417..9651014a79b61 100644
--- a/net-mail/notmuch/notmuch-0.18.2.ebuild
+++ b/net-mail/notmuch/notmuch-0.18.2.ebuild
@@ -15,7 +15,9 @@ HOMEPAGE="http://notmuchmail.org/"
 SRC_URI="${HOMEPAGE%/}/releases/${P}.tar.gz"
 
 LICENSE="GPL-3"
-SLOT="0"
+# Sub-slot corresponds to major wersion of libnotmuch.so.X.Y.  Bump of Y is
+# meant to be binary backward compatible.
+SLOT="0/3"
 KEYWORDS="amd64 x86"
 REQUIRED_USE="
 	python? ( ${PYTHON_REQUIRED_USE} )
diff --git a/net-mail/notmuch/notmuch-0.19-r1.ebuild b/net-mail/notmuch/notmuch-0.19-r1.ebuild
index 5ae253ce6cb7a..656236ca8c701 100644
--- a/net-mail/notmuch/notmuch-0.19-r1.ebuild
+++ b/net-mail/notmuch/notmuch-0.19-r1.ebuild
@@ -15,7 +15,9 @@ HOMEPAGE="http://notmuchmail.org/"
 SRC_URI="${HOMEPAGE%/}/releases/${P}.tar.gz"
 
 LICENSE="GPL-3"
-SLOT="0"
+# Sub-slot corresponds to major wersion of libnotmuch.so.X.Y.  Bump of Y is
+# meant to be binary backward compatible.
+SLOT="0/4"
 KEYWORDS="amd64 x86"
 REQUIRED_USE="
 	nmbug? ( python )