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There are three major characteristics of picks that work together to determine what they sound like when applied to mandolin strings: point shape, thickness, and the material they're made from.
Rounded points tend to produce a darker tone than sharp ones, and also make it easier to get a good smooth tremolo (see glossary). But rounded points can be a bit more difficult to use in terms of accuracy. Many beginning players prefer to start with a sharper point, which makes it easier to "aim" at the correct course of strings.
Thicker picks also tend to produce a darker tone than thinner ones, but they can be more difficult to hang onto. There's a common misconception that you can play faster with a really thin pick, but that's just not true. At high speeds, an overly flexible pick won't have returned to a straight position before its time to hit the next note. But thin picks are very nice for strummed rhythm work.
The type of plastic (usually) that a pick is made out of has a couple of different properties that affect tone. First is flexibility, which is pretty much parallel to the effect of thickness. Then there's the "graininess" of the material. Delrin, for example, is a very smooth, almost greasy feeling plastic which produces a fairly dark tone. Other pick materials that have more "tooth" will produce brighter tonality.
These three factors work together (or against each other) to produce a final result. Some very thick picks will sound brighter than thinner ones, either due to their point shape or the material they're made from, or both.
Another factor in pick choice is the overall size. Larger picks take less "clenching" to hold onto when playing hard, but there are plenty of advocates on both sides of the issue of whether a larger or smaller pick gives you more control.
Bear in mind that pick choice is only one of the many contributing factors to overall tone. The perfect pick for a given situation of instrument, string set, and musical style may fail miserably if you change any of those other elements. If you play different kinds of music or own more than one mandolin, you'll probably have more than one "favorite" pick. If you have a pickup in your mandolin, you may also find yourself preferring different picks for plugged-in vs. acoustic.
So what's a newbie to do? Experiment! Picks are the cheapest of all mandolin accessories, so get an assortment and try them all out. Most music stores will be happy to let you try before you buy -- but bring your own mando, since that's the only one that counts. If you run into another player who's using a pick you haven't tried yet, ask them if you can borrow it for a couple of tunes. The final judgement lies in your own hands and ears: the best picks for you are the ones that feel comfortable and produce the tones you want to hear from your own instruments.
Authored by: Message board member
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