I have read that witnesses in a court are allowed to refer to notes when they are testifying, but, without regard as to why they might feel a need to, are they allowed to take notes while they are being questioned?
(Inspired by the question What remedies can a witness use to satisfy the "all the truth" portion of his oath? here on Law.SE, but I am not looking for any specific country.)
I've never heard of a rule specifically addressing this in the united-states, but I expect most courts would disallow it.
I suspect a court would believe that the note-taking would be a distraction to the witness, whose focus should be on listening to the questions and providing truthful answers. The note-taking may be perceived as a distraction from the testimony for others in the courtroom, as well.
If I were examining the witness, I would probably be entitled to see what the notes say, dragging out the witness's examination. Then the witness would want to take notes about my questions about her notes, and I'd want to see those notes, and you can see how it can get out of control.
By "referring to notes", I don't believe that means notes that have been taken by the witness during the trial. In certain cases, the witness can refer to notes prepared ahead of time to refresh their memory about something. They can't simply read content directly from the notes, though, they have to have an "independent recollection" of the facts. "Notes" here can actually refer to more than just something the witness wrote for the purposes of the trial, it could also be a report, a photograph, or a physical item. As an example, in State v. Brown, 350 N.C. 193 (1999) a witness saw a letter she had previously written, which refreshed her memory about certain statements she had made.
If this is done during testimony, Rule 612 of the Federal Rules of Evidence enables the opposing party to inspect the notes in question, cross-examine the witness about the notes, and enter any relevant portions of the notes into evidence. That means you have to be careful about what notes you use because if you (for example) had notes of a conversation between yourself and your attorney, you would waive attorney-client privilege the moment you use them on the stand.
From a practical standpoint, I'd imagine this makes it very difficult for a witness to take notes while on the stand. Opposing counsel will see you with notes, they'll demand they be turned over for inspection, and now you don't have anything to write on any more.
