A family of Microsoft relational database management systems designed for ease of use.
Further ... when attempting to open a database already opened by a prior user (shared or exclusive makes no difference), the second user's attempt fails, but DOES result in the second user deleting the lock file.
Subsequently when the first user closes and reopens the database, no lock file is created (at least not in the same location as the database), yet the second user still CANNOT open the database - even though no lock file exists.
This is unexpected.
Greg