This is Aquila's AQ-87U string set for tenor ukulele, with four red strings included. The tuning is GCEA, with a regular high G. The red formula strings allow the strings to resonate at lower frequencies to get a fuller sound. These are different formula string to the single Aquila red series.

The copper content is varied in each string, meaning the guage of the strings is similar on each (the variation in sound is achieved by making the strings different density, rather than making them thicker or thinner). In this way, the sound is much clearer and brighter, particularly on the lower (traditionally thicker) strings.