Jack: StuJ, RichH and BillR have provided the placement info, which is also contained on the telltale package purchased from any marine store.
Sadly, probably half the sailboats worldwide don't have telltales on their sails -- how they set their sails without them is beyond me. I can't and I know what I'm looking for!! In order for me to get that last little bit of effeciency out of the main and jib I need the telltales.
When I'm invited on to a sailors boat to help them with sail trim or a sailor asks me a question by phone, my first question to them is "do you have telltales on your main and jib?". Most of the time the answer is NO. If they indicate they have telltales, my second question is "do you know how to read them?". I devote about 7 pages in my book to reading telltales. I've been on race boats that don't have telltales and they wonder why they are middle to back of the pack finishers. The lack of telltales is my first clue. With the inexpensive addition of telltales and the ability to read them a mate can move up 2 or 3 positions.
I'm at the other end of the spectrum from the mates without telltales -- I have them all over the place. The set I especially like are located on the shrouds. I use that set to get me "in the groove". It's the first set I look at because if I'm not in the groove most adjustments to the telltales are ineffective. I then look at my jib telltales (masthead rig) and the set I concentrate on are the middle ones. Next I check the mainsail telltales and the one I concentrate on is the top one. That's my "twist" indicator. It takes me less time to go through that check process than it took to write this paragraph!!
I think any sailmaker worth his salt should automatically add telltales to the sails they sell but a lot don't. My sail bag always contains telltale material so when I go aboard I have them available in the event the owner does not have them. I use a roll of tomato plant tape!!! Whatever works for me I use. The owner can replace it later, and most do, AFTER they find out how effective they are when setting sail trim.
Here's a quick tip. Suppose you're sailing closehauled and the inside telltale starts to flip - what do you do first? Suppose you're sailing closehauled and the outside telltales starts to flip -- what do you do first? Easy, if the inside telltale is flipping then TRIM IN the jib sheet until it stops flipping. If the outside telltale is flipping EASE OUT the jib sheet until it stops flipping. After they are streaming you can fine tune.
No non-electronic devise has been developed that approaches the sensitivity and effectiveness of mainsail and jib telltales but they're useless if you don't know how to read what they are telling you.
