Sometimes written as lock nuts, they also go by various other names, including half nuts, jam nuts and thin nuts.
Locknuts are used to lock a full nut in position to prevent it from working loose as a result of vibration or other factors.
Locknuts are often mixed up with Nyloc nuts, which are different. Nyloc nuts are self-locking nuts that feature a nylon collar in the top portion of the nut that locks the nut in place.
BA thread (British association) is a rarely used thread that was once commonly found on small instruments and models. Still sparingly used in some industries such as metering equipment manufacturing and optical equipment.