I’ve always been curious about the postures in martial arts forms where both feet are together, because these postures don’t look very martial at all. In fact, it’s hard to imagine why you would want to use a stance like that in a fight, and yet we find them in a lot of Tai Chi forms:
From: Sun Lu Tang, A Study of Taiji boxing, 1921
From: Long ZiXiang, A study of Taiji boxing 1952
Here’s an example of the posture in application in Taiji performed by my teacher Sifu Raymond Rand:
It seems to be mainly Taiji lineages that have some influence from Sun Lu Tang that do this the most. A lot of people attribute the distinctive ‘feet together’ postures he used to his prior training in Xing Yi, and there could be some truth to this. Xing Yi does have ‘feet together’ postures quite a lot.
Sun Lu Tang showing a selection of postures from , A Study of Xing Yi Boxing, 1915
Of course, the root of Xing Yi is spear fighting, but the modern interpretation of the art is heavily biased towards bare hand training, and this creates a misleading impression. Think about it – if you were at at least one spear length away from your opponent the risk of being tackled to the ground because your feet are together would be greatly reduced. You’re now free to use the power generation advantages that can be gained by letting both feet come together, which is handy when you are holding a heavy object, like a spear.
If you watch this excellent video of Xing Yi spear technique by Byron Jacobs you’ll see that he doesn’t hang out with his feet together all the time, but occasionally he uses the feet together moments for power generation (and of course, also standing on one leg for range advantage and manoeuvrability in a way that makes sense with weapons).
Example feet together transitional posture:
Byron Jacobs of Mushin Martial Culture
In Xing Yi the most famous example of the ‘feet together’ posture is the Half-Step Beng Quan. Here the back foot stepping up to meet the front foot in place creates a powerful closing action of the body, kind of like a door slamming.
From: Selected subtleties of the Xing Yi Boxing art, by Liu Dianchen 
So, is this the origin of ‘feet together’ postures in Taiji forms? Quite possibly. However, there is one more thing to consider. After first learning Xing Yi, Sun Lu Tang learned his Taiji from Hao Weizhen 1849–1920, who learned from Li Yiyu 1832–1892, who learned from one of the Wu brothers, Wu Yuxiang 1812–1880 who had learned directly from Yang Luchan 1799–1872 and also sought out Chen Qingping 1795–1868 who he learned from in Zhaoboa village.
It’s often thought that the distinctive stepping seen in Sun style Taiji, where the back foot is often lifted and brought up close to the front foot, is a consequence of Sun’s prior Xing Yi training. This makes sense as part of the narrative created as part of the Sun Style Taiji brand, which is that he incorporated his earlier Xing Yi and Bagua training into his Taiji style. However, if you look at the Wu (Hou) style he learned, it already had this distinctive stepping in it.
From: Wu Yuxiang style Taiji Boxing by Hao Shaoru
While the feet don’t go completely together as much, if at all, in Wu(Hao) style, they are very close together for a lot of the time. Watch this video for an example of the form in action:
One theory about why this is is that Wu Yuxiang was a member of the Imperial Court at the end of the Ching Dynasty, and was therefore expected to wear traditional court dress, which restricted the stepping.
I think you can see that influence extending into Sun Lu Tang’s Taiji, which makes sense since he learned from this lineage.
Finally, I should note that thought this post I don’t want to create the impression that all the steps in either Xing Yi or Taiji performed by Sun Lu Tang are small or restricted. He also had plenty of wider postures in his arts too, for example.
However, compare it to postures found in other styles of Taiji whose practitioners didn’t have to wear court dress:
Chen Ziming for example: