Okura Yusoki A Series Manual page 199

11. Only the products laid at the corners of red doublet have been mated so far.
However, other than that, the rest of the products need to be mated using the
functions at top area (C). The details are introduced later on.
11.1 Mate any products along external lines which do not mate with red doublets.
(At section 13.1, you have already experienced with the same case.) When
two products next each other in other direction are laid, click on [Outside] at
[Mate with other dir product]. The below is a example of two products to
mate with red doublet by itself as the default is set to click on [Outside].
Magnified
A next product followed
by one at corner of red
doublet
Other direction
each other
13-9
A product at
four corners of
red doublet