What opus, K, and BWV numbers mean in music

The numbers after the title of a classical piece are catalog numbers. They tell one piece apart from the many others a composer wrote. “Op.” is short for opus, a Latin word that means work. “K.” marks a piece by Mozart, and “BWV” marks a piece by Bach. The number helps you find the exact piece you want.

Opus numbers

An opus number is a number given to a piece or a set of pieces, most often when the music was published. Beethoven’s Symphony No. 5 is Op. 67.

They follow the order of publishing. A low number is often an early work and a high number a late one. The order is not exact, because some pieces were printed years after they were written.

One opus can hold several pieces. Then each piece gets a second number. Beethoven’s “Moonlight” Sonata is Op. 27, No. 2. It is the second of two sonatas printed together.

Some pieces have none. Many composers did not number everything. Beethoven’s “Für Elise” was found after his death, and it is listed as WoO 59. The letters stand for German words that mean “work without opus number.”

“Op. posth.” means the piece was published after the composer died.

Why some composers have letters

Composers who lived before about 1800 seldom numbered their works in a tidy way. Later, scholars made complete lists. Each list has its own letters.

Letters	Composer	Where the letters come from
K.	Mozart	Ludwig von Köchel, who published the list in 1862
BWV	Bach	German words for “Bach works catalog,” published in 1950
Hob.	Haydn	Anthony van Hoboken, who made the list
D.	Schubert	Otto Erich Deutsch, who made the list
RV	Vivaldi	Peter Ryom, who made the list
HWV	Handel	German words for “Handel works catalog”

K numbers run by date

Köchel tried to put all of Mozart’s music in the order it was written. K. 1 is a tiny piano piece from early childhood. K. 626 is the Requiem, which Mozart left unfinished when he died.

So a K number gives a clue to Mozart’s age. Eine kleine Nachtmusik is K. 525, from his early thirties.

BWV numbers run by type

The Bach list is sorted by kind of music, not by date. The church cantatas come first. Organ pieces, keyboard pieces, and pieces for orchestra each have their own stretch of numbers.

So a BWV number tells you what kind of piece it is. It says nothing about when Bach wrote it. The Toccata and Fugue in D minor is BWV 565, among the organ works.

How to read a full title

Take this one: Symphony No. 5 in C minor, Op. 67.

Symphony is the kind of piece. See symphony, concerto, and sonata explained.

No. 5 means it is the fifth symphony this composer wrote.

In C minor is the key, the scale the piece is built on.

Op. 67 is its place in the composer’s published works.

Add the composer’s name, and there is only one piece it can be.

More titles to decode

• Eine kleine Nachtmusik, K. 525: by Mozart, number 525 in Köchel’s list
• Brandenburg Concerto No. 3, BWV 1048: by Bach, among his works for orchestra
• “Spring” from The Four Seasons, Op. 8, No. 1: the first piece in Vivaldi’s eighth published set
• Symphony No. 94, “Surprise”: the nickname was added by listeners, and the number is the one that counts
• “The Trout,” D. 550: a song by Schubert, from the list made by Deutsch

Why the numbers are useful

Composers reused titles. Mozart wrote several piano sonatas in C major. Vivaldi wrote hundreds of concertos. A search for “Mozart Sonata in C” brings up more than one piece. A search for “Mozart K. 545” brings up one piece only.

When a teacher assigns a piece, or when you want the same recording again, write down the number.

Things to do together

Number hunt. Look at the track list of any classical album. Find the letters and say which composer each one belongs to.

Early or late? Pick two K numbers, one low and one high. Play both. Ask which sounds like the work of a child and which sounds grown up.

Make your own. Have your child number their drawings or their own tunes: Op. 1, Op. 2, and so on. Then ask for Op. 3 by name.

Find the composers. Bach, Mozart, Haydn, Schubert, Vivaldi, and Handel each have a page in the guide to composers by era, and all six are on the composer timeline.

For more words that show up on a concert program, see the plain glossary of music words.