I assume you are referring about Pitru Koorcham in Tharpanam. Basically two set of Koorchas made out of 3/5/7/9 darbas are kept in Tharpanam. One represent Pitru varga & other refer to Matru varga.
There will 3 lines of Cut darbas are kept below to highlight vasu, Rudra & Adithya sthanam. Which is only a reference or can be considered as aasanam also. these will be kept east to west & above them the Koorchas will be placed North to South, ending the Sharp edge towards south.
Apart from this, you will have to wear Pavitram made out of 3 Darbas & Kattadarba for samkalpam. Incase of Any puja or samskaaras, the Pavitram will be made of 2 Darbas. Pavitram made of 4 Darbas will be used during the Public welfare programs such as temple Kumbhabhishekam ...etc.