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For acoustic slide playing, the instrument of choice is the resonator (also known as a "Reso-phonic") guitar. These instruments, commonly known by the trade name of Dobro (First patented by R. Dopyera in 1929), where originally manufactured during the 1920's by two companies, Dobro and National guitars. For more on the history of these instruments I'd recommend Wikipedia's entry on Resonator guitars.
They are distinctive for their metallic, haunting, tone. This unique sound is the result of having a metal cone (sometimes more than one) inside the body of the guitar which was originally designed to increase the volume of the guitar. This is usually concealed by a round metallic plate on the front of the guitar. Dobro and National guitars are produced both with timber and metal bodies. The wooden-bodied models generally have a warmer tone. These guitars, both vintage and new models, are expensive to buy. However a number of companies in Asia have been producing affordable copies for a number of years, of varying quality. Some of these companies include: Regal (originally produced in Chicago but now manufactured in Korea), Epiphone, Samick, Sakura and several other brands. These Asian copies are sometimes very hard to fault, although many suffer from having poor tonal characteristics due to their use of unseasoned timbers. Many of these guitars will improve tonally with age though.
These can be divided into two different types: Single-Cone and Tri-Cone
Single Cone Resonators:
These are made with either wooden or metal bodies and either a "Spider" or "Biscuit" style cone. If you look at the above picture of these two types, you will notice that the Spider style uses a concave (pointing inward) cone, while the Biscuit cone is convex (pointing outward). These variations in design give a very different tone to the instruments.
Generally speaking, metal-bodied types, particularly with a "Biscuit" cone have a more Bluesy tone, while for Bluegrass and Country music the wooden-bodied, Spider-cone style is preferred.
Resonator guitars can have either a traditional round neck, or a square neck cross-section. The square-necked guitars are designed for playing while the instrument is laid out flat on the players lap. They usually have a much higher action (the distance between the strings and the fretboard). This style of playing (and instruments designed for this style of playing) is often referred to as "Lap-steel". Square-necked models also usually have the tuners on the headstock inverted for easier access.
I usually think of these as coming in two different types. The first type is an acoustic resonator that has had a pickup fitted into it. The second type is an electric guitar that has had a resonator fitted into it. Either of these modifications may be the way the instrument was produced in the factory or an after-market modification.
Not just guitars have had resonators added to them! Banjos, Mandolins, Ukeleles, Bass guitars, Violins, Double Basses and even Balalaikas have been given the resonator treatment, here's a few examples...
1930's Resonator advert
Dopyera Brothers 1927 Tricone resonator guitar patent:
Dopyera Brothers 1929 resonator guitar patent: