The Sanskrit word tri means three and kona means corner or angle. Thus "three corner or three angle posture. In this asana the position of the body becomes like a triangle so, it is called as Trikonasana. This posture is also known as the utthita trikona-asana. Utthita means stretched or extended thus this is the Extended Triangle Pose.
Trikonasana stretches the spine, opening the trunk and improving balance and concentration.

Position of Readiness

Stand upon the floor. Keep the legs at about two and a half feet distance from one another. Keep hands at your sides, eyes looking forward. Breathe normally.

Steps

Inhale slowly and at the same time raise both arms straight out from the shoulders parallel to the floor with the palms facing downwards.
Start Exhaling slowly while turning the torso to the left, bend at the waist and bring the right hand down to the left ankle. The palm of the right hand is placed along the outside of the left ankle. The left arm should be extended upward. Both legs and arms are kept straight without bending the knees and elbows. You should compete exhaling by the time you have touched the foot.
Now turn the head upward to the left and gaze up at the fingertips of the left hand. Inhale and return to a standing position with the arms outstretched.
Hold this position for the duration of the exhaled breath. Now you will have to repeat same action on the opposite side.
Start Exhaling slowly while turning the torso to the right, bend at the waist and bring the left hand down to the right ankle. The palm of the left hand is placed along the outside of the right ankle. The right arm should be extended upward. Both legs and arms are kept straight without bending the knees and elbows.
You should compete exhaling by the time you have touched the foot.
Now turn the head upward to the right and gaze up at the fingertips of the right hand. Inhale and return to a standing position with the arms outstretched.

Benefits

This asana has medical value, it corrects the disorder of stiffness in the neck, shoulder joints, joints of the waist etc. The stretching of the body has good effect on the muscles of calves, hand, shoulder etc improving their functioning.
Therapeutic for anxiety, flat feet, infertility, neck pain, osteoporosis, and sciatica
Contraindications/Cautions
Diarrhea
Headache
Helps relieve stress
Improves digestion
Helps relieve the symptoms of menopause
Relieves backache, especially through second trimester of pregnancy.

Note

Persons having complaints regarding their waist and the spinal column should do this Asana under the guidance of Yoga Expert.
The posture can be retained longer by breathing gently through the nostrils rather than holding the breath or you may perform the trikona-asana rapidly that will give it a slight aerobic effect.