I recently received this question from one of my members:
I noticed that some teachers are now cueing to keep the feet “one fist-width apart” instead of “hip-width apart”. Why is that?
Have you noticed what happens when you tell your students to “place their feet hip-width apart”? I encourage you to do that in your next class. Watch the result you're getting from this cue.
Typically, a person with larger hips will have her feet farther apart than a person with a narrower build.
A person with weak legs (or weak lateral hip stabilizers such as gluteus medius/minimus) might also place her feet farther apart because it gives her more stability.
Bottom line, hip-width apart is a fairly subjective cue. There is a lot of room for (mis)interpretation. If you tell a student to position her feet "one fist with apart” you get a more consistent result.
There might still be slight deviations - some students will imagine their fists bigger and some smaller - but that's easy to remedy by placing your fist between her ankles. The next time she will remember very easily because she can use her own hand to check her alignment.
The second reason is anatomical. If you look at the image of a skeleton you'll see that the thigh bones angle inward from the hip to the knee. Our shin bones run pretty much vertically. This means, in anatomical position our feet are actually not hip-width apart. Maybe hip joint width, but most students are not aware where their hip joints are located. The bony landmarks of the pelvis are misleading. The projections at the front of the pelvis (ASIS) are not your hip joints; they are not the bony areas on the outside of the pelvis (greater trochanters), either.
Your actual hip joint is located about 2 inches medially (closer to the midline of the body) and 2 inches inferior (lower) to the ASIS (hip bones). If you have a group of students that is open to receiving anatomical information at such a high level, go ahead and empower them with knowledge.
But to get a more accurate alignment without boring overwhelming your students, stick with the fist.
Hope that was helpful.