This is a very short, brief, inadequate, introduction to playing slide guitar.
A slide is a smooth bar or cylinder made of metal, glass, porcelin, or something else hard. It is touched to the strings of a guitar so that it rests on top of the string and doesn't press down onto the fretboard like you would do with your finger for playing regular guitar. The slide can be slid along the strings to make a continuously changing tone. To get notes like on a normal guitar you place the slide directly over a fret. To play multiple strings at the same time the guitar should have a flat fretboard.
Slide guitar technique came out of Hawaii and has been used for blues music since the 1920s at least. It's also quite popular in country music.
The slide is shaped in a cylinder and slipped over a finger on the left hand. The guitar is held in the normal position.
The slide is often a solid bar and held in the left hand. The guitar is held flat in the player's lap, hence the name. There are guitars (some strangely shaped) specialized for this style of play.
These can have rounded necks for playing bottle neck style or square neck (cross section) for playing on the lap.
These have legs and pedals to further manipulate the strings.
"Open" tunings are often used for slide guitar.
In such a tuning the open strings played together at once to form a chord. So such a tuning is generally used to play in that key. You can find the IV chord above the fifth fret and the V chord above the seventh fret. Of course, the I chord is also at the twelfth fret, in addition to at the open strings. You can get a quick feel for slide guitar (and get drawn in to it) by playing around with just these chords.
D2 G2 D3 G3 B3 D4
E2 A2 E3 A3 C#3 E4
Notice that Open A is just the same as Open G, but each string is one step higher. You could tune to Open G and put a capo on fret 2 to give you Open A. So we can be a bit more abstract and think in scale degrees.
Here's a (crude) diagram of the fretboard in spanish tuning.
5|---|--6|---|--7|--1|---|--2|---|--3|--4|---|--5|
3|--4|---|--5|---|--6|---|--7|--1|---|--2|---|--3|
1|---|--2|---|--3|--4|---|--5|---|--6|---|--7|--1|
5|---|--6|---|--7|--1|---|--2|---|--3|--4|---|--5|
1|---|--2|---|--3|--4|---|--5|---|--6|---|--7|--1|
5|---|--6|---|--7|--1|---|--2|---|--3|--4|---|--5|
o 3 5 7 9 12
Notice also the flat sevens and flat thirds on the third fret. Start sliding around over frets 0, 2, 3, and 5 to notice some bluesy feeling. (watch out for that fifth string though).
You can see in the above diagram where the flat sevenths are. Here's I7 chord (no slide):
O O O O O
===========
| | | | | |
| | | | | |
-----------
| | | | | |
| | | | | |
-----------
| | | | | |
| | | | | O
-----------
| | | | | |
| | | | | |
-----------
| | | | | |
To use that same shape up on the fifth fret is quite a stretch. You would put the slide over the fifth fret and have to reach up to the eighth with your pinky.
Or, you can play "behind the slide". Plant an index finger down on the third fret of the third string while your slide plays over the fifth. This works because the fretted third string can vibrate below the slide. You might have to try a few times before you can get it to work.
===========
| | | | | |
| | | | | |
-----------
| | | | | |
| | | | | |
-----------
| | | | | |
| | | O | |
----------- <-3
| | | | | |
| | | | | |
-----------
| | | | | |
O O O | O O
----------- <-5 (slide here)
To get some contrast you can use these chords too without the slide:
IV IV7 V7
X O O X O X X X O O
=========== =========== ===========
| | | | O | | | | | O | | | | | O |
| | | | | | | | | | | | | | | | | |
----------- ----------- -----------
| | | | | | | | | | | | | | | | | |
| | O | | O | | O | | | | | | O | |
----------- ----------- -----------
| | | | | | | | | | | | | | | | | |
| | | | | | | | | O | | | | | | | |
----------- ----------- -----------
| | | | | | | | | | | | | | | | | |
D2 A2 D3 F#3 A3 D4
E2 B2 E3 G#3 B3 E4
Again Open E is the same as Open D, but one step up.
Here's a (crude) diagram of the fretboard in vestapol tuning.
1|---|--2|---|--3|--4|---|--5|---|--6|---|--7|--1|
5|---|--6|---|--7|--1|---|--2|---|--3|--4|---|--5|
3|--4|---|--5|---|--6|---|--7|--1|---|--2|---|--3|
1|---|--2|---|--3|--4|---|--5|---|--6|---|--7|--1|
5|---|--6|---|--7|--1|---|--2|---|--3|--4|---|--5|
1|---|--2|---|--3|--4|---|--5|---|--6|---|--7|--1|
o 3 5 7 9 12
Here are the slideless chords like those above. Notice that they are shifted down one string.
I7 IV IV7 V7
O O O O O O O O O O X X O O
=========== =========== =========== ===========
| | | | | | | | | O | | | | | O | | | | | O | |
| | | | | | | | | | | | | | | | | | | | | | | |
----------- ----------- ----------- -----------
| | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | O | | O | | O | | O | | | O | | O
----------- ----------- ----------- -----------
| | | | | | | | | | | | | | | | | | | | | | | |
| | | | O | | | | | | | | | | | | O | | | | | |
----------- ----------- ----------- -----------
| | | | | | | | | | | | | | | | | | | | | | | |
There is an interesting relation ship between the "Spanish" and "Vestapol" tunings. Notice that these two tunings share a pattern for five strings. The high five for Spanish is the same as the low five for Vestapol.
Here's a diagram for a seven string guitar. This would be an open G tuning. Shown is the key of C major.
g|---|--a|---|--b|--c|---|--d|---|--e|--f|---|--g|
d|---|--e|--f|---|--g|---|--a|---|--b|--c|---|--d|
b|--c|---|--d|---|--e|--f|---|--g|---|--a|---|--b|
G|---|--A|---|--B|--C|---|--D|---|--E|--F|---|--G|
D|---|--E|--F|---|--G|---|--A|---|--B|--C|---|--D|
G|---|--A|---|--B|--C|---|--D|---|--E|--F|---|--G|
D|---|--E|--F|---|--G|---|--A|---|--B|--C|---|--D|
o 3 5 7 9 12
Here it is again, but with scale degrees for G major instead of note names (letters). The scale degrees make it more abstract. This could theoretically be any key. Notice that the low six strings are in the Spanish pattern and the six high strings are in the Vestapol tuning.
1|---|--2|---|--3|--4|---|--5|---|--6|---|--7|--1|
5|---|--6|---|--7|--1|---|--2|---|--3|--4|---|--5|
3|--4|---|--5|---|--6|---|--7|--1|---|--2|---|--3|
1|---|--2|---|--3|--4|---|--5|---|--6|---|--7|--1|
5|---|--6|---|--7|--1|---|--2|---|--3|--4|---|--5|
1|---|--2|---|--3|--4|---|--5|---|--6|---|--7|--1|
5|---|--6|---|--7|--1|---|--2|---|--3|--4|---|--5|
o 3 5 7 9 12
The top six strings form the Vestapol pattern, familiar in Open D and Open E. The bottom six strings form the Spanish pattern, familiar in Open G and Open A. So many riffs in one tuning can be done one string up or down on the other tuning. Similarly chord patterns can be moved up or down one string to play in the other tuning.