Musical notes in music, a

MUSICAL NOTES
In music, a note is a symbol denoting a musical sound. In English usage a note is also the sound itself.
Notes can represent the pitch and duration of a sound in musical notation. A note can also represent a pitch class.
Notes are the building blocks of much written music: discretizations of musical phenomena that facilitate performance, comprehension, and analysis.^[1]
The term note can be used in both generic and specific senses: one might say either "the piece 'Happy Birthday to You' begins with two notes having the same pitch", or "the piece begins with two repetitions of the same note". In the former case, one uses note to refer to a specific musical event; in the latter, one uses the term to refer to a class of events sharing the same pitch. (See also: Key signature names and translations.)

The note A or La

Names of some notes
Two notes with fundamental frequencies in a ratio equal to any integer power of two (e.g., half, twice, or four times) are perceived as very similar. Because of that, all notes with these kinds of relations can be grouped under the same pitch class.
In European music theory, most countries use the solfège naming convention do–re–mi–fa–sol–la–si, including for instance Italy, Portugal, Spain, France, Romania, most Latin American countries, Greece, Albania, Bulgaria, Turkey, Russia, Arabic-speaking and Persian-speaking countries. However, in English- and Dutch-speaking regions, pitch classes are typically represented by the first seven letters of the Latin alphabet (A, B, C, D, E, F and G). A few European countries, including Germany, adopt an almost identical notation, in which H substitutes for B (see below for details). Byzantium used the names Pa–Vu–Ga–Di–Ke–Zo–Ni (Πα–Βου–Γα–Δι–Κε–Ζω–Νη).^[2]
In traditional Indian music, musical notes are called svaras and commonly represented using the seven notes, Sa, Re, Ga, Ma, Pa, Dha, and Ni.
The eighth note, or octave, is given the same name as the first, but has double its frequency. The name octave is also used to indicate the span between a note and another with double frequency. To differentiate two notes that have the same pitch class but fall into different octaves, the system of scientific pitch notation combines a letter name with an Arabic numeral designating a specific octave. For example, the now-standard tuning pitch for most Western music, 440 Hz, is named a′ or A₄.
There are two formal systems to define each note and octave, the Helmholtz pitch notation and the scientific pitch notation.

Accidentals
Letter names are modified by the accidentals. The sharp sign ♯ raises a note by a semitone or half-step, and a flat ♭ lowers it by the same amount. In modern tuning a half step has a frequency ratio of ¹²√2, approximately 1.0595. The accidentals are written after the note name: so, for example, F♯ represents F-sharp, B♭ is B-flat, and C♮ is C natural (or C).

Frequency vs position on treble clef. Each note shown has a frequency of the previous note multiplied by ¹²√2
Additional accidentals are the double-sharp  , raising the frequency by two semitones, and double-flat  , lowering it by that amount.
In musical notation, accidentals are placed before the note symbols. Systematic alterations to the seven lettered pitches in the scale can be indicated by placing the symbols in the key signature, which then apply implicitly to all occurrences of corresponding notes. Explicitly noted accidentals can be used to override this effect for the remainder of a bar. A special accidental, the natural symbol ♮, is used to indicate a pitch unmodified by the alterations in the key signature. Effects of key signature and local accidentals do not accumulate. If the key signature indicates G♯, a local flat before a G makes it G♭ (not G♮), though often this type of rare accidental is expressed as a natural, followed by a flat (♮♭) to make this clear. Likewise (and more commonly), a double sharp  sign on a key signature with a single sharp ♯ indicates only a double sharp, not a triple sharp.
Assuming enharmonicity, many accidentals will create equivalences between pitches that are written differently. For instance, raising the note B to B♯ is equal to the note C. Assuming all such equivalences, the complete chromatic scale adds five additional pitch classes to the original seven lettered notes for a total of 12 (the 13th note completing the octave), each separated by a half-step.
Notes that belong to the diatonic scale relevant in the context are sometimes called diatonic notes; notes that do not meet that criterion are then sometimes called chromatic notes.
Another style of notation, rarely used in English, uses the suffix "is" to indicate a sharp and "es" (only "s" after A and E) for a flat, e.g., Fis for F♯, Ges for G♭, Es for E♭. This system first arose in Germany and is used in almost all European countries whose main language is not English, Greek, or a Romance language (such as French, Portuguese, Spanish, Italian, and Romanian).
In most countries using these suffixes, the letter H is used to represent what is B natural in English, the letter B is used instead of B♭, and Heses (i.e., H ) is used instead of B (although Bes and Heses both denote the English B ). Dutch-speakers in Belgium and the Netherlands use the same suffixes, but applied throughout to the notes A to G, so that B, B♭ and B have the same meaning as in English, although they are called B, Bes, and Beses instead of B, B flat and B double flat. Denmark also uses H, but uses Bes instead of Heses for B .