# Why are G and H used for feedback block diagrams?

The symbols $G$ and $H$ are typically used for the forward gain and feedback fraction in negative feedback block diagrams like this:

The choice of $G$ is, I suppose, an obvious one, meant to be mnemonic for gain.

But $H$ I don't get. Is it just because it's the next letter in the alphabet and maybe $F$ was already taken? Or is there perhaps a historical reason that might lead one to see it as a less arbitrary choice?

Note: I understand these symbols are not universally used in these roles. I kind of grew up on $A_o$ and $\beta$ myself, but as I understand it, $G$ and $H$ are more common in the control systems world and do have some advantages over other choices. Certainly conforming to a convention has its own benefits, whatever that convention is.

• Hysteresis? Heuristic? – Ignacio Vazquez-Abrams Aug 17 '16 at 5:18
• Hmm. I'll guess that Harold Black popularized it and managed to sneak his first initial in there, besides. ;) Of course, I've no clue at all. – jonk Aug 17 '16 at 5:22
• $H(\cdot)$ is commonly used to refer to the transfer function of e.g. n LTI system: en.wikipedia.org/wiki/Transfer_function – Math1000 Aug 17 '16 at 6:06
• @Math1000: Hmm, that's right, I hadn't made that connection. I wonder if that has to do with it, like the transfer function of the feedback network or something. Although in a block diagram of any complexity there are a lot of transfer functions around :). And then of course, it raises the question of how $H$ was chosen for that role :) – scanny Aug 17 '16 at 6:18
• Do you have an example because I see something different i.e. H is used as in H(s) for the overall transfer function. – Andy aka Aug 17 '16 at 7:42