Thread is wound on the spool two ways and depending on how it is wound will dictate whether you put it on a vertical or horizontal spool. If the thread is wound on the spool in a round and round (straight wound or stacked) fashion use a vertical spool. If the thread is wound in a cris cross fashion then use a horizontal spool or a thread holder so the thread comes off the top of the spool when being unwound. My older machines only have a vertical spool spindle but I think when they were made thread only came wound in straight wound fashion.
Here's what I'm talking about.
https://auribuzz.wordpress.com/2016/...urifil-spools/