Every rod blank has a spline, think of it as the backbone of the rod. The spline is made by the extra thickness (overlap) of graphite cloth as the cloth is wrapped onto the mandrel.
The desire is to align the spline on each of the rod blanks. You will also attach the guides along the spline.
To find the spline you’ll need to mark the location on the rod blank. Wrap a little bit of masking tape onto both ends of the blank a couple inches from the end of the blank.
Next you get to “roll” the the blank and feel for when the blank “Jumps” in your hand. With one end of the blank in you hand and the other on a table cloth to prevent damage. You flex the blank and roll it to feel the blank jump (bump) up. This is the spline.
Once you mark the spline on each of the blank sections I like to test myself by trying to identify the spline with my eyes closed.
Often times it is difficult to find the spline on the butt section. If this happens just try your best. The butt section on a 4 piece blank is pretty short and only flex’s a little when casting.