Mainline Mergers are those places associated with BBHs where the lines of two (or more) incoming directions merge into one outgoing direction. There are various methods to build these. A brief overview can be found in this blog post, and many examples can be seen in the Junctionary.

The size of mergers is usually in the form x+y->z, which means that x lines from one direction and y from another direction are merged to z outgoing lines. If only one number is given (like "LL merge" for two lines), it is the number of outgoing lines.