The trembulo is an instrument similar to a tiple

Tiple

Tiple is the Spanish word for treble or soprano, is often applied to specific instruments, generally to refer to a small chordophone of the guitar family. A tiple player is called a tiplista.-Colombian tiple:...

, but smaller. It is thought to have been first developed by Portuguese luthiers around 1600. It seems likely to have developed from the 4-course renaissance guitar.

It has four courses of double nylon strings (probably originally gut). All the courses are tuned in octaves, giving a unique sound, somewhat like a tiple

Tiple

Tiple is the Spanish word for treble or soprano, is often applied to specific instruments, generally to refer to a small chordophone of the guitar family. A tiple player is called a tiplista.-Colombian tiple:...

, but softer due to the nylon strings, and somewhat more like a lute. It is believed to have evolved from the Baroque guitar

Baroque guitar

The Baroque guitar is a guitar from the baroque era , an ancestor of the modern classical guitar. The term is also used for modern instruments made in the same style....

 or Vihuela

Vihuela

Vihuela is a name given to two different guitar-like string instruments: one from 15th and 16th century Spain, usually with 12 paired strings, and the other, the Mexican vihuela, from 19th century Mexico with five strings and typically played in Mariachi bands.-History:The vihuela, as it was known...

, suggested by its slender shape. It usually has a scale length of between 380 and 450mm, but some have as long as 500mm. It is carved out of a single block of wood, like jarana

Jarana

The jarana jarocha is a guitar-shaped fretted stringed instrument, from the southern region of the state of Veracruz, Mexico. Typically strung with 8 strings in 5 courses, usually arranged in two single outer strings with three courses between. The strings are usually nylon, although they were gut...

s and charango

Charango

The charango is a small Andean stringed instrument of the lute family, 66 cm long, traditionally made with the shell of the back of an armadillo. Primarily played in traditional Andean music, and is sometimes used by other Latin American musicians. Many contemporary charangos are now made with...

s. Usually a light wood is chosen for easier vigorous playing such as fast strumming. For the same reason, wooden (or nowadays plastic) pegs are used instead of geared metal tuners.

Usually the trembulo has four double courses, but there are also variants with three double and one single for the lowest (seven strings) and two single and two double (6 strings). The highest courses are always double. It is the lower ones that are sometimes single.
The trembulo was taken to the New World. Sometimes a trembulo can have a keyhole or a mandolin shape, but these are less common.

Trembulo fusão

The trembulo fusão is a modern version of the trembulo, no longer made from a single carved block of wood, but constructed like a guitar or ukulele. The name means fusion trembulo, so named because of its use in Jazz Fusion. Unlike the traditional trembulo, it usually has two double courses and two single courses, for a total of six strings. Small guitars are sometimes converted to and from Trembulo Fusaos in Brazil.