But the form need not necessarily be circular. The idea of comprehension may be metaphorical, as in expressions like ‘rtu-chakra’ (the round of the seasons) ‘nakshatra chakra’ (the collection of stars) and ‘nadi chakra’ (the arrangement of the arteries).

The three words, Chakra, Mandala and Yantra are in fact synonymous. But in popular imagination, each connotes a characteristic feature that distinguishes it from others. For instance, Chakra suggests a circular form, while the ‘mandala’ may be figure of any shape (usually square). While both chakra and mandala are usually linear representation, Yantra a three dimensional model. It must be noted that there is also a three dimentional model of Sri-chakra, called ‘meru’. Whatever it is called, the instrument is a sphere of influence, a consecrated ground, an arena for the play of thoughts, feelings and forces both inside and devotee and outside him. It is an instrument that is employed to activate energies, stimulate thoughts, harmonize feelings and co-ordinate inner and outer forces.

The significance of the prefix ‘Sri’ in the expression ‘Sri-chakra’ denotes that the instrument employed is ‘auspicious, beneficent, salutary, conducive to prosperity. ‘Sri’ is the name of mother goddess who rules over the universe (tvam sristvam iswari). She is so called because all living beings depend upon her for being, for happiness, for the fulfillment of their destiny. The conception and employment of ‘Sri chakra’ are relevant only in the framework of an esoteric discipline, known as ‘Sri Vidya’, which is variously interpreted as ‘the vidya of the nature of Sri’ (the ultimate benefit, viz: ‘mukti’), the ‘vidya that yields ‘sri’ (prosperity) ,the vidya of ‘sri’, the vidya and sri, in the sense of path and goal. The tantrik texts speak of ten ‘vidya’ or cultic goddesses whose worship is commended for health, happiness, wealth and welfare here and liberation from phenomenal bondage hereafter. The ten divinities are classified to 1.the extra-ordinary ‘vidyas’ (maha-vidyas), Kali and Tara 2.the ordinary ‘vidyas’, Shodasi (or Tripura), Bhuvaneswari, Bhairavi, Chinnamasta and Dhumavati and 3. ‘Adept vidyas’ (siddha vidyas), Matangi, kamala and Bhagalamukhi.

The extra-ordinary vidyas presuppose on the part of the devotee great rigor, austerity, persistence and detachment. The practice is filled with grave risks. The ‘adept vidyas are likewise to be propitiated by the would-be adepts and therefore involve rituals of a kind that the common man would find extremely arduous and hazardous. The ‘ordinary vidyas’ are suitable for ordinary aspirants and they are safe. Each of these vidyas has a characteristic form and particular ‘dhyana, mantra, kavacha and other details of tantrik ritual.

The Structure of Sri-Chakra: There is a two-fold way of looking at the ‘yantra’. If we start from the uter-most enclosure (avarana), which is in the form of a square (called ‘bhupura’) and move inward until the dimensionless point (bindu) is reached, the movement suggests progressive implication and contracting of the dimensions leading finally to dissolution in the point. It is described as the way of enfoldment (samhriti). If, on the other hand, we begin with the central dimensionless point (bindu) and move outward until the all-enclosing square (bhupura) is reached, this movement suggest progressive explication and expansion. It is described as the ‘way of unfoldment’ (srshti). In either case, there are NINE enclosures.

The Nine enclosures have characteristic forms, names, meanings, symbolisms and corresponds with the aspects of human constitution. If we follow the ‘way of enfoldment’, the nine enclosures-successively are;

GROUP 1. (Inner Group symbolizing absorption or ‘samhara’)

1. The central dimensionless point (bindu0 representing the transcendental aspect of the mother-goddess, denoted by the secret syllable ‘shrim’. In fact, it is this point (coloued red) that is really the ‘Sri-Chakra’. Everything else is only a manifestation of aspects thereof. The point is identified with lalitha or Rajarajeswari, the chief deity of the Tantrik cult.

2. The primary triangle with its apex downward and coloured white immediately around the central point. It may be seen that this triangle does not intersect with any other triangle and stands independent in structure. Other triangles in the yantras are formed by extending the three sides of this primary inverted triangle. The triangle is feminine in character, is said to represent the three fundamental manifestations of the mother-goddess: Kamesvari (presiding over the Kamarupa-pitha, symbolizing the moon, and representing creation), Vajreswari (presiding over Purnagiri-pitha, symbolizing the sun, and representing preservation) and Bhaga-malini (presiding over Jalandhara-pitha, symbolizing the fire and representing dissolution). The three corners of the triangle stand for the three ‘peaks’ (kuta) of the fifteen-lettered ‘mantra’. The triangle itself is regarded as the abode of the mother-goddess (kama-kala)

3. The figure of eight-corners (ashta-kona) surrounding the primary triangle consisting of eight triangles, coloured Red.

GROUP II (middle group, sumbolising presernation of sthiti)

4. The figure of ten angles (called ‘inner’ or antardasara)consisting of ten triangles, coloured blue, surrounding the eight-coloured figure.

5. Another figure of ten angles (called ‘outer’ or bahirdasara) surrounding the above, also consisting of ten triangles, coloured red.

6. The figure of fourteen angles (consisting of fourteen triangles) (called chatur-dasara) surrounding the above.

GROUP III (outer Group, symbolizing extension of srshti)

7. The ‘eight-petalled lotus’ or the circle with eight petal-like projections, red in colour, on the outside (called ashta-dala-padma) enclosing whole of Group II (which in turn encloses Group I)

8. The sixteen-petalled lotus (shodasa-patraka) or the figure with sixteen petal-like projections on the outside, surrounding the above: and

9. The square field (chaturasra) in which all the above units are positioned. It is imagined to be the ‘earth-stretch’ (bhupara), all inclusive, bound by three ramparts.

Of the above units, units 2 to 6 are regarded as Sakti-aspects and the unit 1 and the last three Siva-aspects. While the above nine constitute the main pattern of Sri-chakra, it is usual to introduce between 9 and 8, three concentric, circles (known as ‘tri-vrtta), representing the three worlds (bhu, bhuvah and svar), the three ‘gunas’ (sattva, rajas and tamas) and the basal conter (muladhara) in the living beings…..

Besides the forty-three triangles involved in the design of Sri-chakra, there are also twenty-four ‘sandhis’ (where two lines meet) and twenty-eight ‘marma-stanas’ (where three lines meet), which have their own significances. During the worship ritual, these spots also receive attention. The meeting of two lines indicates the union of Siva and Sakti, while the meeting of three lines suggests the explicit presence of harmony between Siva and Sakti (described as ‘samarasya’)

The background philosophy explains that the bindu is Sri or Maha-Kameswari, and initial manifestation is ‘nada’ or sound (half-syllable or crescent moon). Thereafter, the primary triangle (trikona) takes shape as the organ of generation of the universe (YONI). The bindu is Kamesvara, the gound of the universe: the ‘trikona’ is Kamesvari, the mother of the universe. The union of these two is Sri-chakra, which represents the entire phenomenal pattern including the underlying unitary principle.

see also: http://www.srividya.org/Srividya/Svtsweb/apps/homeMenu.php 

SOURCE: http://www.geocities.com/purnayoga/glimpses/srichakra025.html  

All that escaped conception's narrow noose
Vision descried and gripped; their seeing thoughts
Filled in the blanks left by the seeking sense.
High architects of possibility
And engineers of the impossible,
Mathematicians of the infinitudes
And theoricians of unknowable truths,
They formulate enigma's postulates
And join the unknown to the apparent worlds.
Acolytes they wait upon the timeless Power,
The cycle of her works investigate;
Passing her fence of wordless privacy
Their mind could penetrate her occult mind
And draw the diagram of her secret thoughts;
They read the codes and ciphers she had sealed,
Copies they made of all her guarded plans,
For every turn of her mysterious course
Assigned a reason and unchanging rule.
The Unseen grew visible to student eyes,
Explained was the immense Inconscient's scheme,
Audacious lines were traced upon the Void;
The Infinite was reduced to square and cube.
Arranging symbol and significance,
Tracing the curve of a transcendent Power,
They framed the cabbala of the cosmic Law,
The balancing line discovered of Life's technique
And structured her magic and her mystery.
- Sri Aurobindo : Savitri(II.11).