RuneScape Classic Wiki
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The main RuneScape Wiki has an article on: Lobster certificate

A Lobster certificate is a certificate that is obtained by exchanging five lobsters with Niles at Draynor Village market, Seth at Brimhaven or Orven at Fishing Guild. Lobster certificates are stackable and each one can be exchanged back for five lobsters by talking to their respective certer.

Because lobsters are not stackable and players can only trade twelve items per trade screen, they are often exchanged for certificates so that they can be traded easier and faster.

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