Śatapatha Brāhmaṇa ("of a hundred paths") 8.6.2.3

8.6.2.3

tasya śiro gāyatryaḥ | ātmā triṣṭubho 'nūkaṃ jagatyaḥ pakṣau paṅktayo 'thaitāsāṃ kakubhāṃ catvāri catvāryakṣarāṇyādāyāticandasyupadadhāti sā sāticandā eva bhavati gāyatrya itarāḥ sampadyante saiva gāyatryaśītiryā bṛhatyaḥ sā bārhatī yā uṣṇihaḥ sauṣṇihyatha yadvaśo yadardharcau yadaindrāgnaṃ yadāvapanaṃ tadaticandā atha yannado yatsūdadohā yatpadanuṣaṅgā yatkiṃcātrānuṣṭupkarmīṇaṃ tadanuṣṭubhaḥ

Gâyatrî verses are the head thereof, Trishtubh verses the body, Gagatî verses the spine, and Paṅkti verses the wings; and of each of those Kakubh verses he takes four syllables, and adds them to the Atikhandas: that is just (what makes) that Atikhandas (excessive metre). The others result in Gâyatrîs: this is just that fourscore of Gâyatrîs, the Brihatîs (make up) the Bârhata one, and the Ushnihs the Aushniha one. And as to the Vasa hymn, the two half-verses, the Aindrâgna (hymn), and the insertion, they are Atikhandas; and as to the Nada-verse, the Sûdadohas, the pâda-appendages, and whatever Anushtubh matter there is, they make up the Anushtubhs.