By default your mouse will form a single crosshair with one vertical and one horizontal line. However, when you have two axes belonging to the same atom type (e.g. in a carbon-carbon correlation spectrum) it is really useful to have a double crosshair (with two vertical and two horizontal lines) which will trace equivalent points on either side of the diagonal. In order to set a double crosshair mouse, go to Windows in the main menu and select Window. When you click on a window in the upper part of this pop-up, you will see the axes displayed in the lower part. One of the columns is headed Panel Type. By default these are all set to be different, so for a 13C-13C window, they will be called C1 and C2. If you set the panel types to be the same, then you will obtain a double crosshair mouse. I find it useful to go through all windows and make sure that all panel types are C1, N1 and H1 and then you will always see a double crosshair mouse in diagonal spectra and you will always see equivalent mouse lines in all other windows, too.