It pays to check the loom before threading so any changes can be made ahead of time.
These are an example of typical wire heddles. Enlarge the photo to see the direction of the angle of the eye. These are installed for right handed threading. The eye slants from front to back, left to right. Because of the direction of the slant, it is more difficult to thread by someone who is left handed.
By flipping the heddle end to end, you can change the threading direction to left handed. The angle slants in the opposite direction (front to back, right to left).
It pays to watch which direction the eye is facing when putting heddles on the loom. When I was helping a friend thread a new-to-her loom, one shaft had the heddles backwards with a left handed angle, which made it awkward for threading and slowed the process. I have noticed it on a couple of my looms also. I think it is worth the time it takes to fix them and will do that the next time the looms are empty.
Here are a few heddles from some of my looms. From the left, the first one is an inserted eye heddle. The second is a wire heddle. The third is a stamped metal heddle and the one on the right is a Leclerc repair heddle. They all have some differences to distinguish front and back or right and left handed. They don't come designated right or left. That is determined by how they are installed.
The inserted eye heddle has a slight bend near the top and bottom. Flipping it side to side will not change the eye angle. To change from right to left, the heddle must be flipped end to end. The bends on the ends won't really help much for determining which end is up, so the best thing is to hang one end loop on something stiff and see which way the eye angles. The chenille wire works well because the heddles don't slip off the wire.
The other wire heddle is like the inserted eye heddle above, with nothing to distinguish the top from the bottom. Hang them and adjust the eye angle.
The stamped heddle is the easiest to determine left and right, because the top and bottom loops are different. One end is straight.
The other end has a wavy top. Line up the wavy tops in the same direction and the eyes should all line up correctly. Decide if left or right threading is what you want and install with the wavy side up or down, depending on your choice. Just make sure they are all the same.
This the end of the Leclerc repair heddle. I love them and have several.
They slip over the heddle bars easily.
Just make sure the eye is facing in the proper direction for threading, just like a normal heddle.
Many weavers are not aware of these differences and can't figure out why threading the loom doesn't go as smoothly as they would like. Take the time to fix the eye direction one time and have years of easy threading. It will be worth it!