I have never used Hysys. I am attempting to decode a material balance generated with Hysys, however, and some of the chemical names are throwing me a little.

What is 22-mbutane?
How about 23-mbutane?

This is probably just something simple. I know butane itself only has two isomers, and I've already identified those earlier in the list.

I did check, but I couldn't find this addressed previously; perhaps it is too simple a question and those of you experienced just know what these abbreviations stand for.