The conventional wisdom is to test what size or shape works best and what is recommended by the manufacturer. From our experience the shape that works the best for a bumper head are round trimmer lines, ranging from 2.4mm to 3mm. This is because the smooth surface does not get stuck as easily in the feeding mechanism. Some customers tell us that they prefer the square lines however. Twisted trimmer lines tend to be too slippery and can easily overfeed or be pulled from the bumper head when cutting so are not recommended.