A certificate (also referred to as a cert) is a tradable and stackable item equivalent in value to five of the equivalent non-stackable item. Certificates can be converted into their equivalent items (and vice-versa) for free by various non-player characters (referred to as "certers"). Only certain commonly-traded items can be converted.
Many players prefer to trade with certificates in order to complete the trading process easier and faster, though it takes a significant amount of time to trade the items for certificates and vice versa. Only 12 non-stackable items can be added per trade screen, but 25,769,803,764 stackable items can be added per trade screen (assuming that each 12 are a different item and the player doing the trading is very, very rich).
The certificates system was originally introduced on 8 December 2001 with an update. At this time, only fish (both raw and cooked), bars, and ores could be converted into certificates. Following the introduction of pay-to-play, the system was expanded, with two more ore and bar certers in Zanaris, log certificates, additional fish certificates (such as shark certificates), and miscellaneous certificates. The system was replaced by the "bank note" system in RuneScape 2 with the justification that they made trading easier than in RuneScape Classic.
There are many different types of certificates which include items that are traded in great volume.
Certer located in: Draynor Village (Niles), Brimhaven (Seth), or Fishing Guild (Orven)
Certer located in: Catherby (Owen) or Fishing Guild (Padik)
Certer located in: Draynor Village (Giles) or Zanaris (Jinno)
Certer Located in: Draynor Village (Miles) or Zanaris (Watto)
Certer located in: East Ardougne (Chuck)
Certer located in: Yanille (Sidney Smith)