Zipper Terms:

Zippers come in toothed, coil or invisible configurations.  The larger the number, the larger the tooth or coil.

Below Picture, top to bottom:
Black with Metal Teeth: #15 Buffed & Polished
Yellow lace:  #3 coil on laser cut tape
Rainbow on Lame tape: #7 coil
Yellow:  #5 molded tooth

Zippers can come separating (a jacket zipper), closed bottom (a dress zipper), closed top & bottom (need to be sewn into a purse, pillow, dog bed on both ends), or by the yard which can be a closed bottom,  or closed top & bottom zipper.

Toothed:  #3, #5, #8, #10, #15, #20.  The number denotes approximately how wide the teeth are across when closed.  Materials for teeth are metal (aluminum, brass, titanium), molded tooth (plastic or resin), or rhinestone (embedded in plastic molded tooth). Usually on a polyester tape but some of the metal zippers come on cotton tape that can be dyed easily.

Coil: #3, #5, #7, #8, #9, #10.  Again the number is approximate width of the zipper when closed.  Coil looks like a spring that has been stretched out & stitched to a polyester twill tape. Coil is usually the strongest because so much area on the actual zipper is touching the other side of the zipper.  It is more flexible than a toothed zipper so a good zipper to go around curves.

Invisible Zippers: #2, #4, #6.  

These zippers are now made only from nylon coil but I have heard that originally they were made in metal tooth. The zipper coil folds to the back of the zipper to hide the actual zipper coil.  Because of the bend in the tape, these zippers are more fragile than a standard coil or toothed zipper. 

#2 - use in light weight fabrics with very little pressure on the zipper. This is probably the size at your fabric store. 

#4 - medium weight fabrics or where you need a little more security, ie. bride's maid dresses, pillows, duvet covers. 

#6 - Vera Wang uses this size in custom wedding gowns, costuming, slipcovers & heavy weight fabrics for pillows or duvet covers. It is 6mm across the closed coil.  You can find this other places but it is 5mm across & the filament is smaller in diameter, hence a weaker zipper.

Separating Zippers or Jacket Zippers: These zippers come completely apart with a box & pin configuration at the bottom of the zipper.  Look at your jacket & observe the bottom box & pin that allows you to take the jacket off.  

You normally see these as a molded tooth & packaging in JoAnn's says "sport" or "Vislon".  Sport is normally a Talon zipper (manufacturer) & Vislon is a name brand for YKK.  Generically they are a #5 molded plastic tooth.  

Separating or Jacket Zippers or Open Bottom zippers can come with 1 or 2 pulls: 1 pull zips up from the bottom or 2 pulls allow you to open the bottom of your jacket to sit comfortably. 
I stock SEPARATING zippers in 
#3 MOLDED TOOTH for baby gear and robes, 
#3 COIL separating zippers for robes & other light weight items, 
#3 METAL TOOTH for fashion applications (not very strong),
#5 MOLDED & COIL for medium weight jackets, sleeping bags, & leaders for quilting machines, 
#5 METAL toothed zippers for light weight leather jackets or hoodies, 
#8 MOLDED tooth for outdoor gear with a larger pull for gloves,
#8 METAL tooth for just a bit more beefy zipper than a standard #5 metal tooth that has a tendency to pop over time,
#10 METAL toothed zippers for motorcycle vests/jackets & Carhartt jackets
#10 MOLDED tooth for marine or outdoor applications, & 
#15 MOLDED tooth for costuming (zipper & tape is 3" across!)

Invisible vs. ALMOST Invisible Separating Zippers:
ALMOST invisible separating zippers are actually #3 coil with the pull put on backwards so that you only see the tape, not the coil when sewn in. These are much stronger than invisible because they don't have the bend in the tape.  I stock these in 12", 14", 30" & 40" for clothing.  

I now stock #5 coil almost invisible separating zippers in black for jackets.  You will see this zipper on high-end ski coats.

Real Invisible SEPARATING zippers are very high tech!  I stock them in #4 on mesh tape & #6 on a twill tape.  #4's are great for blouses or light weight jackets.  #6's are good for heavier jackets.  Both weights (4mm & 6mm) are great for costuming!

Water-proof Zippers
Used in clothing, bags for cameras, marine applications. They are not 100% water-proof but close.

I stock 1 separating zipper in "water resistant" black in 24" & YARDAGE to replace in backpacks.


Name Brand vs. Generic Zippers
I have traveled the world looking for the best zippers for my projects and am passing my findings & knowledge on to you. It's not about the name on your zipper tab, but how it works! There are HUNDREDS of zipper manufacturers in the world & most have great quality. YKK has a big name in the U.S. but their zippers are no longer made here, but in China or Japan or India.  I purchase some zippers from YKK but have found better technology in Vietnam, Israel, Poland, Hong Kong, India and the U.S.A. 

What size zipper for my project?

The smaller the number, the smaller the zipper BUT not necessarily less STRONG!  Picking your zipper is about frequency of use, WHERE the zipper is used and type of fabric.

     #4 or #6 invisible.  I also stock reversing pulls that ride on the Back of the zipper tape so you can use a heavier zipper (#3 or #5 standard) with the look of an invisible.

     I use either a #3 standard coil or a #4 invisible by the yard for pillows made out of light or medium weight fabrics.  If I am doing commercial work, then I usually use a standard coil as it will go through higher temperatures in a dryer & take rougher handling (rushed maids) than an invisible.  If the pillows are over 24" or in upholstery weight fabrics, I tend to use #6 invisible just to be on the safe side.
     Just from using zippers over the past 40 years of sewing, my opinion is that a #6 invisible is about as strong as a #3 standard coil.

Duvet Covers:
     Duvet covers are just big pillows!  The zipper is usually applied at the foot of the cover in the underside fabric.  If the cover is going to be washed frequently, always use a #3 standard zipper.  I usually don't use a #5 standard because the #3 is very strong but supple.  The #5 doesn't bend as easily.

If you are only going to launder your duvet cover infrequently then consider an invisible zipper.

I choose my zipper by the weight of my fabrics:
     Cotton sheeting or silk: #3 standard or #4 invisible zipper
     Upholstery fabric or velvet: #3 standard or #6 invisible zipper

     You need a SEPARATING zipper for jackets.  Use a #3 for children's jackets, #5 for light weight adult jackets, #8 or #10 for leather jackets.
     1-way SEPARATING opens just from the top.
     2-way SEPARATING also opens from the bottom of the jacket so you can open up the bottom of the zipper to sit comfortably.

Boat or Windowseat Cushions:
     Use #5 standard coil by-the-yard.  It is nylon coil on polyester tape, which is inherently UV protected.  #5 is very strong but will still go around the corners of a cushion.  Will stand up to lots of washings or waves.

Sail Covers: 
     #10, #15, #20 molded zipper.

Zipper Heads:

Your zipper head will tell you what sort of zipper you have (coil, metal, plastic toothed).
Look at the back of your zipper head.  There MIGHT be a number & this MIGHT be the size of your zipper unless its just a mold mark.

From Left to Right below:
Square Shoulders: A Metal Zipper. The "8" means it is an 8mm size.
Pointy Head: Usually a #5 or #7 COIL Zipper
Rounded Head: Usually a #3, #5, #9, #10 COIL Zipper
Rounded Shoulders, Flat Top: Usually a MOLDED Plastic Zipper. #3, #5, #7, #8, #9, #10, #15, #20
Crown Top: This is usually a #3 or #5 Molded Plastic Zipper.  These you will find on your jacket.