The yajnOpaveeta comprises of three threads, and each thread is in turn made up of three fine threads (strands). The three threads that form the yajnOpaveeta represent the 3 guNas, 3 vEdas, 3 varNas and the 3 lOkas.
By the order of the Lord the abhimAni dEvatas of these (3 gunas, 3 vEdas, 3 varNas and the 3 lOkas) reside in the three threads.
tAvallOkAstrayO varNAstrivRut sUtram samAshrayan
tat graMthimAshrayattArAstrimAtO nAdasmyutaH
tat graMthyagram cha sAvitrI vEdamAtA shivAj~jayA ||
Each of the three threads of the yajnOpaveeta in turn is made of 3 fine strands, totally numbering to 9 strands for one yajnOpaveeta. The 9 strands (of a single yajnOpaveeta having three threads) have the following deities:
1st strand: OmkAra
2nd strand: agni
3rd strand: bhaga
4th strand: sOma
5th strand: pitRu
6th strand: prajApati
7th strand: vasu
8th strand: dharma
9th strand: vishvEdEvatas
tRuteeyO bhagadEvatyachaturthaH sOmadEvataH
paMchamaH pitRudEvatyaH ShaShThachaiva prajApatiH
saptamO vasudEvatyO dharmashchAshTamaM Evacha
navamaH sarvadEvatyaH ityEtA nava dEvatAH ||
The yajnOpaveeta should not go either below or above the navel. It has to be changed every year on the upAkarma day and also after any ashaucha (mailige).
Bachelors should wear one yajnOpaveeta, where as a minimum of two is compulsory for gRuhasthas. The second one represents the extra duty a householder has to perform. (The 2nd one is required since the wife cannot wear a yajnOpaveeta and the husband wears one for her, since the husband is the guru of the wife.). gRuhasthas can wear even 3 or 4 yajnOpavItas, one representing the uttareeya and the other for shrEyas.