"Indravajra", "Upendravajra", "Bhujangavijrmbhita", "Sragdhara" are some of the metres in devotional and other poetical works. Some of them are intricate and only highly gifted people are capable of composing them.
As mentioned earlier, the foot of a stanza with eight syllables Anustubh. With nine syllables it is "Brhati" and with ten "Pankti". "Tristubh" has eleven syllables and "Jagati" twelve. We have a 26-syllable metre ("Bhujangavijrambhita") which belongs to the category of "Utkrti". Beyond this is "Dandaka" of which there are several types. The metre in which Apparasvamigal's Tiru-t-tandagam is composed is related to this metre.
Some metres have beautiful names. In poems composed in a certain metre the flow of words reminds of a playful tiger lunging forward; the metre is appropriately called "Sardulavikridita". "Sardula" means tiger; "vikridita" is playfulness. (This metre, belonging to the category of "Atidhriti", has 19 syllables). Each pada in it is divided into 12 and 7 syllables. Adi Sankara's Sivanandalahari is partly in this metre (a number of verses from the 28th stanza onwards). The initial verses of the part called "Stuti-satakam" of the Muka-Pancasati (which is a hymn to Kamaksi) are in this metre. The concluding one hundred verses, "Mandasmita-satakam", are entirely in this metre. "Bhujangaprayata" is the name of another metre which suggests a snake(bhujanga) gliding along. Our Acharya's Subrahmanya-bhujangam is in this metre. It belongs to the Jagati type with 12 syllables a foot, divided into six and six as in
Ma-yu-ra-dhi-ru-dham
Ma-ha-va-kya-gu-dham
Our Achrya's Saundaryalahari is in the Sikharini metre. It has 17 syllables in each pada. (It belongs to the category of Atyasti) The 17 syllables are divided into two parts of six and 11. The "Padaravinda-satakam" of the Muka-Pancasati is in this metre. The metre called "Sragdhara" suggests a flow of words breaking through the floodgates of poetry. It has 21 syllables (belonging to the "Prakrti" class) and each pada has three sets of seven syllables. Our Acarya's hymns to Siva and Visnu (describing them from foot to head and from head to foot - padadikesanta and kesadi-padanta) are in this metre.
I mentioned "Indravajra" first. It belongs to the Tristubh category with 11 syllables in each pada. Another 11 syllables metre is "Upendravajra". A mixture of both is "Upajati": Kalidasa's Kumarasambhavam is in this metre.
All these metres belong to the post-Vedic period and are employed in poetical works as well as in hymns to various deities. "Gayatri", "Usnik", "Anustubh", "Pankti", "Tristubh" and "Jagati" are Vedic metres.
"Gayatri" is a maha-mantra, the king of mantras. A mantra is usually named after the deity it invokes. "Siva-Pancaksari", "Narayana-Astaksari", "Rama-Trayodasi": in each of these the name of the deity as well as the number of syllables in the mantra are combined. The deity for Gayatri is Savita. Gayatri is the name of the metre also. The metre too, one should infer from this, has divine power expressed through the sound and tone of a mantra.
Gayatri, unlike most other mantras and slokas, has only three feet. Each foot has eight syllables and altogether there are 24 syllables. Because it has only three padas or feet it is called "Tripada-Gayatri". There are other Gayatris also. The first Vedic mantra, "Agnimile", is in the Gayatri metre.
(The 24-syllable Gayatri metre used in poetry and non-Vedic hymns has four padas, each of six syllables. Usnik has also four padas, each of seven syllables).
So far I have spoken about metres based on the number of syllables, that is without worrying about whether a syllable is long or short. In prosody the long and short syllables are called "guru" and "laghu" respectively. Poems that make no distinction between "short" and "long" are called "vrttas": those based on mantras are called "jati". In the latter type, a short syllable is one mantra and a long syllable is two mantras. Instead of the number of syllables what matters here is the number of matras.
The "Arya-satakam" of Muka-Pancasati is in the Arya metre. Amba, as Arya, belongs to the most plane; so it is proper that the verse used in singing her praises should also belong to an equally high order. That is why they are in the Arya metre, which is based on matras and not on the number of syllables. if you go by the number of syllables you are likely to be misled into thinking that the metre differs from verse to verse.