In music, the septimal semicomma, a seven-limit semicomma, is the ratio 126/125 and is equal to approximately 13.79 cents ( play (help·info)). It is also called the small septimal comma and the starling comma after its use in starling temperament.
Factored into primes it is:
Or as simple just intervals:
Thus it is the difference between three minor thirds of 6/5 plus a septimal minor third of 7/6 and an octave (2/1). This comma is important to certain tuning systems, such as septimal meantone temperament.
A diminished seventh chord consisting of three minor thirds and a subminor third making up an octave is possible in such systems. This characteristic feature of these tuning systems is known as the septimal semicomma diminished seventh chord.
In equal temperament 
- Haluska, Jan (2003). The Mathematical Theory of Tone Systems, p.xxvi. ISBN 978-0-8247-4714-5.