"A number of other safeguards may also be built into voting systems to help protect the security and secrecy of ballots, for example: Stamping ballots with an official stamp specific to the polling station when they are given to voters or before they are placed in the ballots box; Having one or more polling-station officials sign the back of the blank ballot when it is given to the voter; Use of numbered ballot stubs to monitor the number of votes in a ballot box; Use of a stamp rather than a pen to mark ballots; Use of heavy paper for ballots so that marks cannot be seen through the back of the ballot paper; Printing ballots with watermarks or other devices to make them harder to counterfeit.”"
OSCE (ODIHR): Election Observation Handbook (Fifth Edition), p.43

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Election Observation Handbook (Fifth Edition)