The GCN relies on two types of alerts: notices and circulars. Notices are machine-readable alerts, which are distributed in real time; they typically include only basic information about the event. Circulars are brief human-readable alerts, which are distributed (typically by e-mail) with a low latency but not in real time; they can also contain predictions, requests for follow-up observations from other observatories, or advertise observing plans.