A thaat is a musical mode in Hindustani music which always has seven notes (excluding the repeated tonic) and is considered the basis for the system of organizing and classifying ragas in North Indian classical music. The notion of thaat, and the ten basic categories, were created by Pt. Vishnu Narayan Bhatkhande in the early decades of the twentieth century.
Each one of the several traditional ragas is based on, or is a variation of, ten basic thaats, or musical scales or frameworks. The ten thaats are Bilawal, Kafi, Khamaj, Asavari, Bhairavi, Bhairav, Kalyan, Marwa, Poorvi, Todi; if one were to pick a raga at random, it should be possible to find that it is based on one or the other of these thaats.
Bilawal: Bilawal is the most basic of ten Thaats. It is basically the Major scale/Ionian Mode. Bilawal has a very common tonal character, nowadays Bilawal is not used as a Raaga. Bilawal is also used as the primary lesson for vocal or instruments.
Kafi: Kafi is one of the oldest scales of IMC. Its intervals are portrayed as the basic scale of the Natyashastra . In ancient and medieval times, Kafi was considered as natural scale. Pure Raaga depicted from this Thaat is a late evening Raag and said to convey the mood of spring time.
Khamaj: The Raagas of this particular Thaat are very romantic. These are mostly used in the form of light classical thumris, tappas, horis, kajris etc. Its visual descriptions in the existing texts are sensuous and the Raag Khamaj is considered to be a 'flirtatious' Raag. There is another school of thought which guesses that in the past, Khamaj scale found its way in Ch'in music of the late medieval China.
Asavari: Asavari is full of tyag or devotion. It has the mood of renunciation and sacrifice as well as pathos. It is best suited for late morning.
Bhairavi: Bhairavi is named after the feminine aspect of the extraterrestrial life force, which is personified as a spouse of Lord Shiva. Therefore, it is visualized in female form. It is also very powerful and filled with devotion and compassion. Bhairavi is actually performed early in the morning in a peaceful, serious and occasionally sad mood. Traditionally Bhairavi is used as the last item of a program, for its exceptional detail of sentiments and also for its broad tonal arrangement.
Bhairav: Bhairav is one of the names of Lord Shiva. It can be depicted as the masculine form of Bhairavi. In this form, Shiva is portrayed to be very powerful as a naked ascetic with matted locks and body smeared with ashes. And the Raag Bhairav also shows similar characteristics in composition and tone.
Kalyan: The word Kalyan literally means “Good Luck.” And the Raag is believed to be a seeker of blessing and tonally very soothing. It is usually performed in the evening or in the beginning of a concert.
Marwa: Marwa illustrates the mood of sunset. It considered that Raagas of Marwa awakens the darkness in people or also give a feeling of anxiety or serious expectations.
Poorvi: Poorvi is very deep & serious. It has a quiet and mysterious character and is performed at the time of sunset. Poorvi did not appear in the literature before the 16th century.
Todi: Todi is believed to be the king of all Thaats. Todi is usually visualized as a beautiful woman, holding a Veena (A plucked instrument), with deer around her standing in a lovely forest. Todi represents the mood of charmed adoration with a loving sentiment. It is traditionally performed in the late morning.