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I have a computer with two sticks of DDR4 RAM, and someone just gave me some old RAM that's DDR3. Can I use both in the same computer?

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    Please provide the specifications for your motherboard. – Ramhound Nov 27 '19 at 17:24
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Can I use both in the same computer?

Your system is only compatible with DDR4 memory. The layout of the pins on a DDR3 modules are completely different to that of a DDR4 module. You would be unable to install the DDR3 modules into your motherboard, if you attempted to do so, you would damage the module and/or the motherboard itself.

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DDR4 SO-DIMMs have 260 pins instead of the 204 pins of DDR3 SO-DIMMs, spaced at 0.5 rather than 0.6 mm, and are 2.0 mm wider (69.6 versus 67.6 mm), but remain the same 30 mm in height.

Source: DDR4 SDRAM

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No.
They use different connectors, different rules to activate and refresh the stored values, and different voltages, so even if someone could make a physical adapter, it still would not work. It's very, very rare that a motherboard would have both kinds of connectors.

If you will provide the make and model of your motherboard, laptop, desktop or server, we can advise whether you use DIMMs or SO-DIMMs.

DDR3 for desktop and server PCs use a 240 pin connector; DDR3 SO-DIMMs for laptops have 204 pins.

DDR4 for desktop and server PCs use a 288 pin connector; DDR4 SO-DIMMs for laptops and some Mac desktops have 260 pins. Source1 Source 2 Micron data sheet

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Yes but only with some special mainboards and you can't use both RAM types at the same time. DDR standards are never compatible with each other, because for big improvements you need to avoid the legacy issues. Mainboards with 2 types of RAM modules are rare and only for some transition period. For example there were mainboards with both DDR and DDR2, or DDR2 and DDR3 modules

The are also mainboards that supports both DDR3 and DDR4 like the Asrock B150M Combo-G

But generally they don't worth the effort and it'll be easier and cheaper to buy new RAM sticks

See also

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