On a stringed instrument, a break in an otherwise ascending (or descending) order of string pitches is known as a re-entry. A re-entrant tuning, therefore, is a tuning which does not order all the strings (or more properly the courses) from the lowest pitch to the highest pitch (or vice versa).

Most common re-entrant tunings have only one re-entry. In the case of the ukulele, for example, the re-entry is between the third and fourth strings, while in the case of the Venezuelan cuatro it is between the first and second strings.

Instruments usually tuned in this way include:

Instruments often (but not always) re-entrantly tuned include:

Instruments not usually considered re-entrant, but which have common re-entrant alternate tunings:

The standard tunings for instruments with multi-string courses, such as the twelve string guitar, eight string bass, or Colombian tiple are not considered re-entrant, as the principal strings of each course are ordered from lowest to highest.

Ukuleles other than the tenor and baritone are most commonly tuned in re-entrant fashion; the tenor often is as well, and occasionally the baritone. These conventional re-entrant tunings G₄–C₄–E₄–A₄ are sometimes known as high 4th tunings or high G tuning.

Non-re-entrant tunings, also known as low 4th tunings, exist for these instruments.