They are unusual in that they were probably the most "scientific" design of screw, starting with 0BA at 6.0 mm diameter and 1.0 mm pitch and progressing in a geometric sequence where each larger number was 0.9 times the pitch of the last size.
BA threads are still used in some precision instruments, such as optics and moving-coil meters, relays etc.
In the UK, 2BA threads are commonly found in the old imperial British Engineering Standards Association (BESA) conduit boxes, and 4BA threads in light switch and socket back boxes which are still in use (but not installed) today.
BA sizes are specified by the following British Standards: The angle of the thread is 47.5°[4]: 2 and the depth of thread is 0.6 times the pitch with rounded tops and bottoms.
The thread angle is different from that used by Whitworth (55°), US Unified threads (60°) and ISO Metric (60°) so BA fasteners are not properly interchangeable with Whitworth or metric ones even when the pitch and diameter are similar enough that they can be screwed together (e.g., although 0BA appears similar to M6×1mm, the male and female surfaces do not mate properly for bearing a load).