A great source for Yoga Postures by Mark Giubarelli.
YOGA POSTURES 
ASANA YOGA POSES YOGA POSITIONS TADASANA
Tadasana Downward Dog Cobra pose Headstand Shoulderstand Warrior 1 pose Warrior 2 pose Triangle pose Half Moon pose child pose Cat Pose
Trikonasana
Translation: Triangle Pose 
Step the legs apart. Turn one leg 90 degrees. Place the lower hand on the mat, if possible, and extend the other up. The left one is shown opposite. Then slowly enter the yoga posture by placing the hand down on the leg or on the mat (The hand can be placed on the inside or outside of the feet. You can also grab the big toe). Vertically align the arms with the shoulders. Stretch downwards and upwards with that arms.
Tips
Do not over rotate the neck. If the neck muscles hurt look down then up. Repeat these mild rotations to ease the muscles then look back up again. Maintain a straight alignment in the neck and look sideways instead of up if the neck muscles are still aggravated.
Balance is sometimes hard especially when entering the posture. Use the pinky toe and the big toe for stabilizing the body when entering and while performing this posture.
Weight usually shifts on the surface area of the feet. In the front foot the weight and forces will shift towards the position of the hand. If it is on the outside force will shift to the outside of the foot and the opposite if the hand is placed on the inside. Push energy through the pinky toe or big toe to balance this problem. Placing the hand on top of the leg is another great Triangle Pose.
Pick the big toe up in the back foot for a moment. This will balance the weight on the foot of the lagging (back) leg.
Benefits Triangle Pose
The hamstring muscles are stretched in the leading leg (the biceps femoris, semitendinosus and the semimembranosus). Hip abductors are stretched (the gluteus medius and gluteus minimus) and muscles on the outer part of the hip the ilio-tibial tract are also stretched.
Muscles involved in the external hip rotation in the back leg are the quadratus femoris, gemellus Superior and inferior, obturator internus and externus and the piriformis, they are all strengthened. When they are used the inner leg muscles in the back leg become stretched. See more about Triangle pose in Beginners yoga.
The Trapezius muscles are well stretched during this exercise. See more about yoga poses and back muscles.
They are the pectineus, adductor brevis, adductor magnus, gracilis tendon of the psoas major and further up the pelvis the iliacus on the outer part. Free yoga printable image sequences.
Vinyasa Yoga Sequencing and Flowing for Triangle Pose
Vinyasa Yoga postures to transition from and to:
Yoga Teacher Tips for Triangle Pose
It is nice to exhale into this yoga posture. You may want to instruct this first then the basic actions involved in entering the pose.
Tadasana and Triangle pose
“The neck is a common area for pain. If the posture is being held for a long time give options to relieve any discomfort. This will reduce suffering in the practitioners and sooner or later Trikonasana can be held without any neck discomfort.”
