The thét nhạc is recognized as the most ancient ca trù modality. One can find out only in this modality all the rules related to the five-tune musical scale named as Nam, Bắc, Huỳnh, Pha and Nao. After the book titled Việt Nam Ca trù biên khảo (“Studies on the Vietnamese ca trù”), the existence of this modality was back to the Lê dynasties in XV siècle.

In the Lê dynasty, a ca trù performance was done first with an “overture” during which the chief of service gave the order to musicians to show off their musical instruments – the original term was “thiết nhạc” with the “thiết” as a verb, “to present”, and gradually the Sino Vietnamese “thiết” would change into pure Vietnamese “thét”.  This kind of ca trù overture was done during representations through consecutive dynasties. This modality stressed on the play of musical instruments, thus the role of lyrics remains only the accompanist of the musical part.

Tiếng dương tranh
Đàn ai một tiếng dương tranh

Chưng thủa ngọc ô, đàn não nùng chiều ai oán
Xa đưa tiếng nhạc thiều tâu
Vẳng nghe chuông gióng lâu lâu lại dừng

Bóng trăng thanh nguyệt dãi thềm lan
Tiếc thay mặt ngọc thương ai
Vậy là đêm đông trường
Lá thu ngô đồng rụng
Một lá thu bay, hơi sương lọt mày
Ngồi nghe tiếng đàn

Nguyệt dãi tường lầu, đồng vọng bóng trăng thâu
Ghẹo lòng thiếu nữ
Gửi bước đường trường
Chày ai nện tương tư sầu.

Humble is its role, anyway the lyrics would keep entirely its specific beauty, of which one can find in an ancient poem from the prior-Lê dynasties expressing the feelings of a young woman during a sun-setting afternoon, in solitude inside her room listening to the angelus from a neighboring pagoda bell, and she devoted herself body and soul to her beloved man.