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In a class diagram when associations are shown between classes, Papyrus currently shows arrows in both ends. As the case with both ends *not* beeing navigable really is strange, the UML specification has a recommendations to in the case of double navigability to remove the arrows. ---Quote from the UML spec (section 11.5.4, page 210): In practice, it is often convenient to suppress some of the arrows and crosses that signify navigability of association ends. A conforming tool may provide various options for showing navigation arrows and crosses. As with dot notation, these options apply at the level of complete diagrams. • Show all arrows and crosses. Navigation and its absence are made completely explicit. • Suppress all arrows and crosses. No inference can be drawn about navigation. • Suppress all crosses. Suppress arrows for Associations with navigability in both directions, and show arrows only for Associations with one-way navigability. In this case, the two-way navigability cannot be distinguished from situations where there is no navigation at all; however, the latter case occurs rarely in practice. ---end quote--- In system modeling we have used this on plenty of diagram and the result is more cluttered diagram due to these lots of arrows. It is proposed that Papyrus follows this practical and convenient recommendation. This makes large and complicated class diagrams easier to read.
An additional comment is that one possibility is to introduce a preference for using double arrows or not when both ends are navigable.
