Separating zippers come either in right or left closure, depending on the country's norm. The U.S. norm is left closure: the pin is on the left and it enters the slider(s) from left to right (from the point of view of the wearer). Of course, that means that the slider is attached to the right side. In Canada, Europe and many other countries, the standard is right closure.
People interested in making their own jacket inserts must pay close attention to the
right/left configuration of their jacket's zipper
because zippers of one kind will not mate with those of the other.
A common scenario is for people in Europe or Canada to own imported US brand jackets equipped with left zippers.
When attempting to purchase matching zippers for these,
they find that their local sewing notion stores only stock right zippers.
Luckily, at least in the case of YKK tooth (Vislon) zippers, the right and left zippers can be easily inter-converted. The manufacturer uses the same tooth tracks and sliders for their right and left zippers and simply assembles them in the opposite configuration for the different markets. This means that you can cut off the top stop of a YKK Vislon tooth zipper, slide the slider off the top, turn it around, reinsert it facing backwards and your zipper is now converted to the opposite hand.
Two-way separating zippers (those with two sliders instead of one) can also be inter-converted in this manner.
YKK regularly tweaks its manufacturing standards, which leads to small differences in the look of the pins, the design of the sliders, etc. but newer versions of Vislon zippers remain compatible with previous ones, provided the sliders look like the one on the right in the photograph. About 10 years ago, YKK changed their tooth zipper design significantly and the new style zippers are not compatible with the old-style ones (left on the photo).
If you are mail-ordering your insert's zipper, you should make sure that you will not be slipped an old-style one that has been sitting in the bin these past 10 years. On the upside, if your jacket has an old-style zipper and you do not want to (or cannot) replace it with a new one, a bit of looking around may turn up a matching old-style zipper.
Inter-converting left and right tooth zippers may also work with tooth zippers of other brands but no attempt has been made to verify this.
Unfortunately, it is not possible to inter-convert right and left separating coil zippers. Since the coil tracks are sewn on top of the backing tape, these zippers are not symmetrical and neither are their sliders. To make an insert for a jacket with a coil zipper, an identical zipper to the jacket's must be purchased. If the national standard does not match the jacket's zipper, the choice is either to mail order the appropriate insert's zipper from abroad or to remove the jacket's zipper and replace it with a locally-purchased one.