OK, we know that Mark Time has you marching in place and Half Step has you marching forward at half the size as a normal step. But, there is confusion out there and I see it constantly.
Mark Time Service Differences
The Army tells us that the foot is brought straight up off the marching surface two inches.
The Marine Corps tells us that the toe comes up two inches and the heel comes up four inches off the deck. This also applies to the Navy, Coast Guard, and Merchant Mariners.
The Air Force tells us that the foot comes up four inches from the marching surface.
In order to accomplish marching in place properly, the ankle of the rising leg must remain in line with the leg that remains in place. The toe leaves the marching surface last and strikes first. We do not Mark Time with the whole foot lifting and striking as one and the ankle does not come in front of or go behind the opposite leg. We also do not stomp. Use your gluteal and thigh muscles to lift AND lower both legs to keep everything above the waist from excessive movement (except arm swing, which is the same as marching forward). You should only have slight movement from side-to-side.
While some who attempt Mark Time have difficulties, Half Step is where we can see all kinds of problems.
Half Step Differences
All step measurements below are made from heel-to-heel.
The Army tells us Half Step, 15″, is half of a full step, 30″. When we march at a full 30″ step, the heel strikes the ground first. Half Step should be the exact same, only shorter: the heel must strike first.
The Marine Corps tells us that Half Step is executed with a 15″ toe-first strike. The heels must end up 15″ apart as the toe strikes the deck and the foot rolls back to flat (heel on the deck). This also applies to the Navy, Coast Guard, and Merchant Mariners. The interesting thing here is that the toe must strike far enough away from the trail foot to have the lead foot heel land 15″ away from the heel of the trail foot.
The Air Force tells us Half Step, 12″, is half of a full step, 24″. When we march at a full 24″ step, the heel strikes the ground first, the same as the Army, only shorter. Half Step should be the exact same, again, only shorter: the heel must strike first.
There is one more thing to address; foot height. Half Step is not an adjusted Mark Time step, you do not raise your foot any higher than necessary. Your steps are only supposed to be high enough to clear the ground/deck.