What optional lights may a sailing vessel display?

A sailing vessel may display two all-round lights in a vertical line as an optional light configuration. This arrangement is typically used to indicate when the vessel is engaged in fishing or when it is making way under sail only. When these lights are shown, they provide clear visibility to other vessels regarding the sailing vessel's status and intention, enhancing navigational safety.

For other light configurations, while they can indicate different statuses, they do not provide the specific indication that the vertical all-round lights do. For instance, one all-round white light alone does not offer enough information regarding the vessel's activity or intentions. Red and green sidelights are mandatory for indicating the vessels’ sideline visibility, but they do not serve to communicate the operational state of a sailing vessel specifically. A single white light, while visible, does not convey the necessary information related to a sailing vessel's activities as effectively as the two all-round lights in a vertical line.