The /shout command allows you to speak more "loudly" with a larger font and spiky border. The shouted message is seen by everyone regardless of their location in a chat circle. No pirate can shout outdoors on colonized islands or inside an inn.
On subscriber oceans, only subscribers are allowed to shout outside on uncolonized islands. On doubloon oceans, greenies are not permitted to do so. Attempting to shout on an uncolonized island without the proper permissions results in the message "Avast! Yer voice is too weak to shout."
Shouting is useful for e.g. those running events who need to be heard above the general chatter, or ship officers giving orders to their crew. Using /shout unnecessarily (such as indoors, or if you are not an officer of the ship) is considered obnoxious (much like spamming) and may result in you being planked.
/shout <message> (/sh)
Before release 2005-03-28, pirates could shout outside on colonized islands.