Open tuning
My guitar-style is built on open tunings. I usually tune to open E or A with different variations such as minor or E6, but there are other variations, which is for me to know and you to find out.
Open C, I often use on recordings but rarely on stage. In standard tuning it is possible to achieve a voicing that resembles open tuning by retuning one string. Many folk-singers, for instance, use a drop D tuning and you can say that it gives them a half Open D tuning at the bottom 3 strings and a “normal” tuning on the treble strings.
The method has always appealed to me because bass-lines and chords are logically and naturally at hand and the open strings give the guitar an “open” piano like sound.

The Bottleneck
I use a glass-bottleneck on my left little-finger. I like the smooth tone of glass and I prefer it to the heavier brass or steel. I have always known about the method of slide-playing, but it was first after I studied Blind Willie Motel, Tampa Red and Robert Nighthawk a second time, I chose to explore this style.
A common denominator for the real bottleneck players is that the guitar, in their hands, becomes another unique instrument, more than a mere effect.

The secret brotherhood 
It is my opinion, that when you experience the possibilities of the open tuned guitar, (which is the basis of the bottleneck and Hawaiian slack-key & country Dobra-styles) there is no turning back. The style has lately become very popular in all genres of popular music and can be heard on a number of albums with many popular artists.
You can tell if the players on these productions are real slide-players or they just grab a bottleneck for the occasion and also if they play on the basis of an open tuning.
This is where we separate the sheep from the goats.
In general you can draw the line where you can tell that the guitar in principle becomes a unique instrument and the same goes for any other tune-able stringed instrument

This is done by the following doctrine:
1 – Open tuning must be present – one or more string re-tuned
2 – The bottleneck is placed on the little-finger (for greater versatility)
3 – Fretted notes can alternately or preferably be played on open strings
4 – Regardless of tuning the ability to play in any key must be present
5 – The ability to accompany one self on the basis of a full arrangement that involves
6 – Disharmonic and ”unused” notes and strings is damped
7 – Vibrato must be un-independent of a given pulse or rhythm