Apart from the Portrait series, which is still being printed, all other Singapore banknotes have a last prefix—the final combination of letters and numbers used before printing stopped for that denomination and design. These last prefixes are often more valuable because they were printed in smaller quantities, sometimes fewer than one million pieces. That makes them rarer and harder to find. Collectors are drawn to last prefixes not only for their scarcity but also because they mark the end of a note’s production. Some seek them to complete a full collection, while others value them for their limited availability.

Related to last prefixes is also a special group called “crossover prefixes”. Crossover prefixes are used when a last batch of banknotes is replaced by a new batch, with the new batch having the same prefix and denomination but a different security feature, signatory, or printing company. These prefixes represent a bridge between two eras of banknotes and are relatively more valued.