The six fundamental postures are right at the beginning of every Astanga Yoga practice. No matter how advanced you are, after five rounds of Surya Namaskara A and B, you start with these postures. Here, you carry the rhythm you have set up in Surya Namaskara A & B in a flow from asana to asana. With this solid basis, you are perfectly prepared to enter whichever series you are practising.
Parivrtta Trikonasana, or 'Revolved Triangle Pose', is usually sequenced just after (as a counterpose to) Trikonasana. You can also use this pose as a standing preparation for seated forward bends and twists.

####Parivrtta Trikonasana Instructions
ekam
Inhale, stepping back with the right foot, extend arms to the side
Dhristi: to the tip of the hand (hastagre)
dve
Exhale, place left hand on outside edge of right foot
Dhristi: to the tip of the foot (padayoragre)
5 breaths, Parivrtta Trikonasana (right side)
Dhristi: to the tip of the hand (hastagre)
trini
Inhale, square up, turn, extend arms to side
Dhristi: to the tip of the hand (hastagre)
catvari
Exhale, place right hand on outside edge of left foot
Dhristi: to the tip of the foot (padayoragre)
5 breaths, Parivrtta Trikonasana (left side)
Dhristi: to the tip of the hand (hastagre)
pacca
Inhale, square off to side (turn)
Dhristi: to the nose (nasagre)
Exhale, stepping to the front of the mat, Samasthitih
Dhristi: to the nose (nasagre)
####Parivrtta Trikonasana Benefits
Stretches and strengthens the legs (thigh, calf and hamstring)
Stretches the hips and spine
Opens the chest to improve breathing
Relieves mild back pain
Stimulates the abdominal organs
Improves sense of balance