Derailleur hanger / dropout replacement : A checklist

Example of a hanger

Bicycle Derailleur Hanger replacement
Replacing a hanger can be a nightmare.
What are the 'rules' when replacing one ?

Here is my 'naive' list for when having to replace your hanger.
My knowledge : 3/10

Is there a universal/generic hanger ?	No. Your hanger will likely be specific to your frame
do the bike makers make spare hangers ?	It seems most don't. CNC machine companies sell copies on Ebay/Amazon etc.
How do I identify my hanger ?	If you know the bike make and model and year. But in the end it seems theVISUAL comparison is the most important bit. It has to look very like a copy on a CNC list of hangers
Are the generic hangers in shops any use ?	Probably only as a 'get me home' hanger, if that. It probably will fit badlyif at all
How many hanger types are there ?	100+ ?
Where should I buy ?	Probably Ebay from a CNC comapny. Check their photo list or send them an email. You could send them a photo of your hanger to help
Try the bike maker ?	You could try
Hanger bolt	If you still have your old hanger bolt you can use that. If not, you'd be best buying the bolt too. This will be an option by Ebay sellers
Price	Expect maybe around £7/$15 without bolt ? Add another £1/$1 for bolt