Swings are performers who do not have one specific role in the show, instead they understudy the ensemble roles. In general theatre terms, swings cover ensemble roles while the ensemble understudy principal roles, leading to a "domino effect" as three or four people move roles to cover one principal absence. However in the case of Cats, certainly in productions with a larger number of swings, the swings often cover principal roles as well as ensemble.
Cats has run between having no swings, but named ensemble characters who can be cut from the show should the performer need to understudy a more prominent role, to running with 4-5 swings, to having more swings backstage than performers onstage - the Hamburg production at one point listed 27 swings to 22 characters onstage.
Some productions of Cats refer to the swings as "Cats Chorus", in other productions the "Cats Chorus" are specifically Booth Singers, who may or may not also perform onstage. Booth singers are backstage, and sing along with the cast onstage to augment the vocals. In modern productions it is common for some, or all of the swings to sing booth when not performing onstage. In older productions the Chorus/booth singers typically covered the vocal heavy roles: Grizabella, Jennyanydots, Jellylorum; Old Deuteronomy, Gus, Munkustrap, Tugger.
The term "Walking Cover" refers to a specific swing track, in which an actor covers the roles of Old Deuteronomy and Gus / Bustopher Jones. These roles are not dance intensive, and the Walking Cover does not generally also cover the dancing roles.
Sometimes in Cats the swings have their own distinctive costumes and character names. The practical application for this is that one performer may cover 4-5 small roles, and rather than have 5 separate costumes he can wear the swing costume for all the roles. This is also particularly useful when a female swing has to cover a male character, or vice versa.
In Australian based productions, the swings appear onstage in certain scenes, and interact with the audience. In UK based productions they only appear onstage while covering a named role, except for on special occasions. The UK Swings often appear in the official photocall as a means of giving the performers their credit due, however these images do not represent the performance. Recent UK productions use the Electra and George (Palladium Pouncival) costumes for swings, and additional swings mix and match elements of extra costumes for photoshoot costumes.
The Oslo 1985 Production named their swing characters "Kvesesta", "Potefar", and "Godnattakatt", as well as crediting a "Young Grizabella/Swing".
In the early years of the Original London production, there was an ensemble of un-named kitten characters, some of whom grew into named roles and others became swing costumes. Characters were cast strictly to height, the adults being notably taller than the kittens. This led to issues of costume fitting when a short swing would need to cover a tall character or vice versa. Each of the six swings had a distinctive costume, but if they have official names, they are unknown. For ease of reference they have been given nicknames by fans.
The Swings usually featured in the photocall for the brochure in the big group shots until the late 1990s. However Summer Strallen, a swing wearing NBQ, featured in the 2001 photos.
"NBQ"This tall girl looks somewhat like a pale Cassandra, she is often called "Greycat" or "Nameless Brown Queen/NBQ" depending on the colouring of her costume which varied from silver to tan. She is often used when the tall swing needs to play a kitten, such as Etcetera or Jemima.
"AJ"This Swing looks similar to Jemima but with much less colour and more white on her body, sometimes called "Almost Jemima/AJ". She has been worn on occasions such as playing Carbucketty.
"Tabbygirl"The last female swing looks rather like Victor, a dark brown Tabby, sometimes known as "Tabbygirl". She is often worn by the swing who covers more vocal heavy / mature roles, so appears more mature than a kitten.
The three male swings are harder to identify, as their unitard markings varied enormously and sometimes looked very like the ensemble boys. Their wig designs however were more consistent, but not necessarily paired with the same costume on all occasions.
"Patches"The first of the boys has very defined patches in his makeup, and his unitard often resembles Admetus however he's kitten height.
"Calico"The second sometimes resembles a male Etcetera, a calico male (which in real cats is a genetic anomaly, Calico cats are 99.9% female), with orange and grey stripes, and a strong orange and a black patch in his bangs.
"Caramel"The last male swing is a tall boy often in rich brown and caramel colours, and can be mistaken for Victor.
The Australian production, and subsequent productions modelled on it, names all the swings, which can cause confusion when comparing to other productions.
The swings, usually listed as "CATS Chorus" (not to be confused with the term referring to booth singers), each has their own costume, name, and personality, and function as on-stage ensemble, filling in large group dances and singing ensemble, unless the actor is required to cover another role, or is off themselves, in which case the chorus character will be cut for that performance.