My Daughter wants to build a guitar body. I got a jig built today. Centerline and guide lines laser etched in. Two steel pins to align the body for two sided milling. I am really happy with the accuracy and repeatability of the Avid Router. My previous experience was on a little router with a ton of backlash.
Fixture, not a jig. 
Yes, I was corrected about that on my YouTube channel, and not as politely. 
My dad was an aircraft mechanic. We rebuilt an aircraft together. He built jigs to drill holes in parts or hold things in correct positions for welding. But like you pointed out, they were to guide tools. He learned long before the days of “fixtures” holding things for cnc. I learned so many things from him!
Nice, We just toured the Boeing museum in Seattle, and the McMinnville one near Portland.
I was looking at the old biplanes (some without skins ) and thinking, “I could build that in my shop!”.
Like I need another hobby…