Gimme` a "T"(pose) Gimme` an "A"(pose). The battle of the Poses.

Ok this has been bugging me for a long time now and it falls into the category of , why are we doing it this way.

The T pose and the A pose.

T Pose : This pose is the standard pose. Arms straight out 90 degreees from the body, hands are palm down. Legs are straight and feet point straight ahead.

Pros :
Easy to setup joints and other rig pieces that function or setup easier or better when limbs are aligned with the world axis. Easier to model into T pose as limbs are once again straight.

Cons :
I will go area by area.

1. Forearm: The forearm is modeled twisted and can cause issues when un twisted during rigging and animation.
2. Shoulder and Clavicle : When modeled in T pose the clavicle is slightly raised and the deltoid is engaged and squashed. This means that you must model the deltoid knowing that it is at its maximum squash as the shoulder only rotates to 90 degrees before the clavicle is engaged. This one is a double edged sword because on larger characters with fat and skin, un squashing the underarm a little can help when modeled in the T pose and puts the arms higher.
3. Groin : How comfortable are you sitting with your legs completely straight? I know I'm not because it squishes my groiny bits together. T pose squashes your inner legs and groin together and also in this pose the skin begins to self collide and must be modeled as such. In modeling collided flesh you flatten out the area that must later be given volume when the legs are spread.

Now onto.

A Pose : Limbs are modeled in a more relaxed position minimising any squash or stretch to any part of the body. Arms are at a 40 degree angle and slightly forward. Elbows slightly bent.

Forearms: This is a biggie as there are 2 ways to model this. As the arm twists just less then 132 degrees we will call the middle point 67.5 and zero is with palms forward and forearm bones completely un twisted

1. Anatomically un twisted: Modeled palm forward this puts the forearm in its un twisted form the Zero degree twist state. The bones are straight and the muscles are straight but the arm geometry will need to twist 132 degrees for the maximum human rotation, thus twisting the geometry more than the next option.
2. Anatomically neutral : This is when you model the arm at the 67 degree mark and slightly twist the forearm but model the geometry straight so that it deforms minimally from median to max/min as it is only going 67 degrees in either direction.

Pros : 

When rigging and deforming the mesh starts from a more neutral state. The mesh must go through less deformation to assume opposite maximum poses e.g. : Arm from straight out in T pose to straight down along the body.

Also this means that the joints need to rotate less to assume certain positions. This means less flipping or other bad joint behavior.

Misc Pros :

Cons : 

Personally the hardest thing is not being able to model straight out, having things in an A pose means making sure things flow at the proper angle the whole time and not letting things get curvy.

Other differences :
Eyes: The eyelids are modeled in a closed position, this is the natural state of eyelids as when open they fold onto themselves and into the brow or cheek. This makes rigging easier as the eyelids line up  perfectly during a blink and not in the hands of the rigger to make the blink work by deforming the eyelid to eyeball line.

Mouth :  Also modeled in the slightly open position so that the upper and lower lip do not touch. This allows for easier rigging of lip collisions and squashing as when the jaw is completely shut the lips push together slightly which is not a neutral state.


All of this is just my opinion because at the end of the day it is art.

Here is a grain of salt.