I once attended a workshop of the Mexican ceramist Gustavo Perez and saw him using a fancy tool, a bamboo stick with 3 knife blades sticking out of it. They were arranged one beside the other, with an interspace of approx. a quarter of an inch. His work often has parallel deep lines in it, and he explained that if he uses a knife, cutting a line, and then want to draw another line just beside it, the first cut closes again because of the squeezing out of the clay. So he made a tool with 2, 3 and also one with 5 blades side by side to get clean cut, parallel lines.
How about you? Are you too making your own tools if you can’t find the perfect one at the suppliers?