Lateral marks and junctions: read the whole arrangement

Lateral marks define the sides of a channel relative to conventional direction. At a junction, a modified lateral mark can also indicate the preferred channel.

Recognising it requires more than a familiar colour: combine region, shape or topmark, principal colour, band and light. The preferred channel is the one designated by the authority; it is not a rating of comfort, destination or priority between vessels.

In Region A port is red and starboard green. The distinctive port shape is cylindrical; starboard is conical.

Pillar and spar presentations are also permitted. Where a distinctive body shape is not used for identification, the corresponding topmark should be carried where practicable.

If fitted, the light has that side’s colour in the region. Ordinary laterals can use different rhythms except the one reserved for modified marks: there is no universal period or group for all red or green marks.

A modified lateral carries a broad horizontal band of the other lateral colour. Its body retains a principal colour and the topmark, if present, retains its lateral identity.

The light uses the principal colour and composite group flashing: two, then one, before repeating. International notation Fl(2+1) distinguishes that organisation from Fl(3), one uniform group of three.

The band is not decoration and does not turn the topmark into two cones.

The preferred-channel name and lateral role can seem opposite, but describe different things. If the mark acts as the port-hand boundary of the preferred channel, that channel lies to its starboard when read in conventional direction.

If it acts as the starboard-hand boundary, the preferred channel lies to port. This is a defined spatial relationship, rather than an instruction to turn whenever the mark appears ahead.

Roles and shapes remain, but colours swap. Preferred channel to starboard: principal green, red band, green can and green Fl(2+1).

Preferred channel to port: principal red, green band, upward red cone and red Fl(2+1). A description changing only the light while retaining the wrong body colour is inconsistent.

The table above represents A; it is not a universal colour table for both regions.

Compare it with an ordinary red lateral: in A both can have a red can, but only the modified one adds the green band and corresponding 2+1 grouping. Lettering identifies the particular mark; the band conveys its junction role.

These are separate information fields. Describing only “red buoy number four” can therefore identify an object without explaining what the modified mark means.

A complete educational record keeps particular identity and system meaning separate.