This is a simple thing really.... and confining the discussion to dual channel ? 2 sticks.
1. Dual channel memory is designed to work in pairs ..... that means ya supposed to by a package of 2.
2. RAM is tested at the factory to make sure they work **in pairs**. What likely happens to RAM that doesn't pass the test ? It's sold as single sticks. So when buying single sticks, there's a fair likelihood that the stick had trouble "playing with others".
3. If you don't buy them together you have no guarantee that it will work with any other stick.
4. if you decide to enter the crapshoot that is adding RAM, you want to try and match as many things as possible including manufacturer, model line, DDR type (essential), series, version number, timings and module supplier. Manufacturer's often switch suppliers over time and may use one supplier for version 2.62 and another for version 3.13
5. The lower the speed and higher the timing, the better chance you have of getting the pair to work. The higher the speed, the lower the timing the lower the yields will be for that model and the harder.
6. The MoBo must be able to support the speeds and timings.
7. The JEDEC spec is for the most part irrelevant because most RAM purchased ... and I am talking in excess of 95% ... is not JEDEC compliant, at least not at the advertised speeds listed on the package. It will have JEDEC compliant settings programmed into the SPD, but the number on the package 98% of the time will be the XMP rating. If you bought a set of DDR3-1600 way back when, that number on the package and the number used in the advertising is the XMP rating. XMP is an Intel standard which, by definition is overclocked and therefore exceeds the JEDEC spec ...
https://www.intel.com/content/www/us/en/gaming/extreme-memory-profile-xmp.html
I have seen 100s of posts claiming that if ya exceed the JEDEC voltage, your system is toast. Intel publishes a huge list or RAM kits that a certified compatible" which are well in excess of JEDEC voltages. No MoBo manufacturer has ever had an issue with listed XMP required voltages when their literature states it supports DDR speeds that require those voltages.
At the time when JEDEC covered up to 1333 @ 1.5 volts, the XMP or advertised rating was 1600 @ 1.65. Over time, JEDEC spec was revised to add faster speeds but again, most of the enthusiast / gaming community has never installed RAM using the JEDEC settings. Back then the appropriate voltage for XMP was 1.65 for these sticks. Over time we saw JEDEC adopt faster speeds and we saw production line improvements deliver lower voltages (down to 1.25) but even in DDR3's waning days.... 1.65 was common for 2133 and up.
Even now we are seeing the same thing with DDR4 about anything over the JEDEC 1/2 is oooh ooh scary scary ....and yet, Intel still has much higher voltages on its certified compatible list.
http://www.legitreviews.com/what-is-the-safe-voltage-range-for-ddr4-memory-overclocking_150115
So while JEDEC goes ony up to 1.2v Intel says:
“1.5v is the absolute max we allow for XMP certifications. However, good DDR4 memory will run at 1.35v up to 3200. Technically, no “safe” (guaranteed) OC over-voltage but 1.35v or lower is best.” – Intel
So, JEDEC is only going to be relevant if you bought some very inexpensive RAM that has no XMP (Intel) or AMP AMD) rating.
That RAM in the CPUz image above was sold as DDR3-2400 on the package....to get that speed you will need to use 1.65 volts. It's been running at 1.71 volts for 3+ years.
The long and short of it is, you want to match what you have now whether it be JEDEC or XMP ratings and settings. If you find yourself with a stick of XMP rated DDR3-1600 RAM @ 1.65 volts and then a later model with JEDEC rated 1600 at 1.5 volts, it would not be worthwhile to attempt to make that pairing. That being said, if I had a 2nd stick for free, I'd try and get bit to work at the lowest common settings.... But I wouldn't invest in paying for one. I'd sell it and buy a pair.