A rectifiable pen was a type of pen used by the Doctor. When used to write messages, the pen would sometimes censor words, which would appear as "[RECTIFIED]".
The Tenth Doctor used a rectifiable pen to write a postcard to his companion Donna Noble on the planet Midnight after he had been abruptly summoned away from their trip to the Leisure Palace Spa there by the Duke of Manhattan and sent on a quest to find a puppy. The Doctor used the pen to get his message to Donna past their sensors. The pen censored part of the message, including details about the puppy and the identity of its owner, and the makeup of the Sapphire Waterfall the Doctor enthused about in the message.
Some time later, the Doctor misplaced his rectifiable pen on the Ood Sphere, and so visited Tiaanamat at Akhaten in search of a replacement. He wrote about it in a postcard he sent to Clara Oswald care of Coal Hill School.
The Thirteenth Doctor later used a rectifiable pen to write a postcard to her companion Yasmin Khan, sent from Darillium, having found it after a Pting got to her other pens. The pen rectified a part of the message where the Doctor commented that she had "absolutely got" to bring someone to the Singing Towers. (PROSE: From the universe with love! [+]Loading...["From the universe with love! (short story)"])