In modern music notation, accidental are generally marked on the left side of a note to indicate that the note's pitch needs to be temporarily changed.
Accidentals are used differently from key signatures. Accidental generally lasts for one measure; Key signature will keep the note(s) sharp or flat until it encounters an accidental or another key signature.
There are five common accidentals:
"Da capo" is an Italian term meaning "from the beginning". It is often abbreviated as "D.C." in sheet music, indicating that the performer should return to the beginning of the piece and play it again upon seeing the "Da capo" sign.
In equal temperament (one of the modern musical tunings), enharmonic equivalent refers to "two notes with the same pitch", but they each have "two different names".
For example, in piano, the black key between C and D can be called C♯ or D♭. Although C♯ and D♭ have the same pitch, different names have different functions depending on the harmony and chord progressions.
A key signature indicates which notes a musician should raise or lower while playing a piece of music, and is denoted with sharp (♯) or flat (♭).
The key signature is typically marked at the beginning of the music, and is generally applicable throughout the entire piece or movement. There may be instances of key changes within the music, in which case a new key signature will be used.
The key signature helps the musician understand the musical structure of the piece. For example, if the key signature is written by 2 sharps, the key will be D Major (or B minor).
"Obbligato" is a musical term that used by composers - the creator of the work. It indicates that the certain element or part is extremely important to the music itself, and should not be omitted or altered.
In music or other art forms, works are often re-arranged, simplified, or even omitted by other musicians (performers or composers) or artists. Therefore, to maintain the integrity of the piece, composers use the term "obbligato" to indicate that the certain part is "indispensable".
The opposite of "obbligato" is "ad libitum", a notation that indicates that the related parts can be changed or omitted according to the needs of the performer or conductor.
An octave is a specific interval or distance between two pitches.
In musicology, an octave is a distance between two pitches. In which, one pitch has a frequency that is twice the other (2:1). For example, a string that vibrates at 880 times per second produces a pitch that is an octave higher than a string that vibrates at 440 times per second.
Because of this "twice as fast" frequency relationship, when we hear two pitches that are an octave apart, we perceive them as being very similar, and therefore, they are given the same letter name. For example, a pitch produced by a string vibrating at 440 times per second is called "A", and another pitch produced by a string vibrating at 880 times per second is also called "A".
Since modern musical notation typically uses seven letters of the alphabet (and accidentals) to represent different pitches, the cycle starts again after the eighth. Therefore, pitches that are named with the same letter are part of the same "pitch class", or simply "octave".
Octaves are used in most of the music systems, such as tonal music (major and minor keys), pentatonic music, and serial music (twelve-tone technique).
Unison refers to two or more voices playing the same pitch at the same time in a musical passage. It creates a sense of unity in the music.