These additional frets facilitate the use of still more scales and modes without retuning. After 200 years it is due to return to the Whitechapel Bell Foundry for retuning. I'd like to apologise to Amen Corner for not retuning the piano afterwards. Additionally fast retuning of the crystal is often employed. The crack and the subsequent retuning gives Big Ben its present distinctive tone. An NA of 0.45 could require retuning of a few percent.