Complex sentence: The original standard was defined for both 5 volt and 3.3 volt cards, with 3.3 V cards having a key on the side to prevent them from being inserted fully into a 5 V-only slot.
Split into simpler sentences: The original standard was defined for both 5 volt and 3.3 volt cards. The 3.3 V cards have a key on the side to prevent them from being inserted fully into a 5 V-only slot.