I have read a couple of articles that have pictures of compounds, but I'm not really sure what they are. My latest HSM shows building a fixture that fits into one and holds a tool for threading. It looks like a large T slot, but is there something inside the slot that locks the base of the fixture? This seems like a piece of common knowledge I am missing.
Do you mean the Compound Slide on a lathe?
This is the little slide on top of the Cross Slide which is mounted on a boss and clamped with teenuts in a circular tee slot. This allows the compound to be moved so it is parallel with the cross slide or parallel with the bed or any angle in between.
It is possible to set the compound at an angle to turn a taper but only short tapers are possible because the travel on the compound is short.
The compound slide is often used for threading and is set at the half angle of the thread. Then when the tool is advanced into the cut using the compound it cuts only on one flank.
An open mind is a virtue...so long as all the common sense has not leaked out.