Let’s talk about how thread is wound onto its spool and what it means for you, your sewing machine, and the quality of your work. The thread sits on the spool either cross-wound or stacked.
The cross-wound thread looks like an X when wound onto the spool, whereas the stacked thread forms neat stacks of fibres parallel to each other.
The cross-wound thread creates the least troubles when you let it flow off its spool in one continuous motion, meaning you should place it horizontally like in the photo above. My example spool is obviously too long, so I’d place it on a separate vertical spool holder next to the machine. If your spool would fit on the horizontal pin, you should match the size of the spool cap to the end of the spool (smaller cap than end) to allow the thread as smooth a motion as possible.
If you place stacked thread on a horizontal bar, you risk the thread to get stuck in one of those grooves that are cut into the spool end (where the stacked thread in my photo is secured right now). Instead, the stacked thread likes to be placed on a vertical spool pin, as shown in the photo. This allows the spool to turn on the pin and the thread to move smoothly. Remember the friction pad between the machine and the spool, though, or you can damage the surface of your machine. My pad is round and some kind of felt, smaller than the spool end displayed above.
This is a small piece of information, but experienced voices say it can make a big difference, so I’ll make sure to keep track of the thread type and spool pin from now on.
Please post questions in the comments and I’ll do my best to help or suggest other sources, if this was too much of a nutshell version for you!