As soon as you place them on the side of a block with no support below

Being a forewarning it's immensely Minecraft Seeds important to either back up your save or take up a new one before loading this mod.

 You might wrap up destroying many of your preferred structures otherwise.
Blocks will stack normally but a good most them are not strong enough to carry up in mid air.

they will land to the bottom.
If you use better blocks like bricks or planks however, blocks with more structure, you'll be able to place them on the side of other less steady blocks.

All you have to to do is hold down the CTRL scroll and key with your mouse or arrow keys. It will bring up 3 more inventory bars to cycle to use as your primary selection.


Alternatively unless you want to circuit the whole bar you can choose individual slots to improve by The Building Bricks Mod breaks up normal blocks into segmented items to be able to create custom shapes and sizes. It also gives you to create stairs and slabs out of nearly any stop type.
See it     Tails Mod  at minecraftio.com/tails-mod/