Guitar is a very versatile instrument. An accomplished guitarist can play strummed (block chord) or picked (arpeggiated chord) accompaniment, solo melody, improvised solos, melody and accompaniment at the same time, or even several lines of counterpoint at the same time. As composer or arranger, you may specify exactly how you want something played (even which fingers of both hands are used), or you may write a fairly sketchy part (melodic line and chords, or just chords) and expect that the guitarist to fill it out for you.
If all you want from the guitarist is a chordal accompaniment of the type that guitars often provide, it is enough to provide the guitarist with just chord names (along with some indication of when to change chords, such as the lyrics, or a staff with measures indicated). You do not have to write out the accompaniment unless you want a particular rhythm, strum, or picking pattern. If the chord rhythm or picking pattern you want is very repetitive, you may wish to write out just the rhythm or pattern and indicate how you want it repeated.
If you want specific notes or a melody from the guitar, of course you will have to write it out. Some guitarists are not comfortable reading common notation; they may prefer that you write the music in tablature . Don't write out very complex parts for the guitar unless you are very familiar with the instrument. Very fast, complex music can be played on the instrument, but only if it is written by someone who knows whether a particular combination of notes and fingerings are easy, difficult, or impossible. If you do write out complex parts that are only easy or possible using a certain fingering, be sure to notate it properly (with fingerings, left hand position, etc.).
Also, some guitar chords are easier to play, and sound better than others. (See above .) Keys that favor open strings (i.e. any key in which the most common chords tend to contain the notes E, A, G, D, and/or B) are preferred by many guitarists. Favorite keys include: G, D, C, A, and E major, and E, A, and D minor. Keys with more than one flat can be daunting and may sound noticeably less resonant. If you are writing in a key that is awkward for guitar, you may want to consider giving the guitar player the option of using a capo and playing in an easier key. This will require offering a transposed version of the chords, but will make your music much more approachable for younger and/or less-experienced players. On the other hand, many players of electric guitar prefer the control over the resonance that barre chords (with no open strings) give them. If this will be the case for your music, key matters much less. If you do offer a capo version of the chord names, remember that a capo can only raise, not lower the sound of the guitar. Since the sound of the instrument has been raised, the guitarist can play chords from a lower (easier) key. (For example, if you want a piece to sound in A flat major, you can have the guitarist capo first fret, raising the sound by a half step, and play the chords - in G major - that would normally sound a half-step lower. For more information and practice transposing, please see Transposition .) Capoing above about the fourth fret tends to give a tinny, out-of-tune sound, so try to find a key only a step or two lower that will give easy-to-play chords.
The guitar is such a versatile instrument that there are many different styles and methods of playing it. Classical guitar, folk guitar, rock guitar, flamenco guitar, and blues guitar are just some of the classifications of guitar music that require very different methods, styles, skills, and even instruments. Most guitarists specialize in just one or two of these areas. If you are writing for a particular player, or want a particular sound, your arrangement will be more successful if you are aware of what is reasonable to ask of that player, instrument, and style.
Bass guitar is not normally a strummed instrument. It is usually given the bass line, and in some groups is considered interchangeable with the double bass. It can be a solo instrument, but write a very light, soft accompaniment when you write bass solos, so that they can be heard.