How do you achieve perfectly straight lines on the side of a cake tier?

I have a plastic ruler that is stored with my baking supplies and is ONLY used with food. For vertical lines, I just lightly imprint the ruler into the side of the iced cake. Horizontal lines are more difficult. I know someone on here uses a laser level to help.

It is best to start with a perfect cut out stripe. I then roll it like a tape, put a "line" of glue onto the cake, and unroll the stripe onto the cake. I then use the back of a butter knife and then push down/up the areas where it is not level.

Ok cool, thanks. I wasn't sure if there was some trick that was used for the flawless lines. Thanks for the input

I eyeball it, straight and symmetrical has never been my thing though.

For a horizontal line, I use dental floss. First, I use my cake-only ruler to measure the same distance up from the cake board. I make a toothpick mark. I do that about every 2" or so, then I take a length of dental floss, pull it taut and touch it all the way around the cake, making sure that it hits all of the toothpick marks. Obviously, this works on buttercream, but not fondant. HTH

I use my cake leveler. I put the cake on the countertop, adjust the cake leveler to the height I want, then slightly indent the cake leveler (wire/blade) into the cake. This gives me a nice even line around the cake that is level to the base. I do this multiple times before putting multiple lines on.

Hope this helps.