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Placement of meshes

Hello.
First of all, I would like to apologize if I make mistakes, I am 15 years old and I am French.
I have a problem :
I want to create a building and I need to assemble walls, but I can’t put them side by side without they merge. For now, I have to manually write the positions by doing calculations with their sizes.
Is there a way to avoid this problem? (keyboard shortcut?)
Thank you in advance.

Regards,
Pro Gramer