What does "sitting flush" mean?

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Hello, I just received all my parts to build my system, and i have been researching on how to install the heatsink on to the cpu. I have been reading and I always see the word "flush" used in sentences like "sitting flush" or "in flush position" What does it mean? BTW, I have a 1.4ghz athlon with a Coolermaster EP5-6I11 fan/heatsink and artic silver 2 compound. Any recomendations? thanks in advance

 

[RustyNale]

It means sitting without a difference of heighth between two different surfaces. In terms of a hs and cpu, as close to perfectly flat, as close together as you can get.

 

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so when they say to be in flush position before connecting the second clip of the heatsink to the cpu base that means to make sure the cpu has been attached to the motherboard and that the cpu and heatsnk are in good contact with the heatsnk well aligned on top? Thanks for your help