Grid
Constructions that you build out of blocks in Space Engineers are known as grids -- in contrast to voxel structures such as asteroids and planets. Grids are made of blocks, blocks are made of components, components are made of materials, materials are made of ores.
Examples
Grids include all forms of vehicles and player-made structures of any size, the smallest being singular blocks. Grids are either static (large Stations) or mobile; mobile grids can be either large (Large Grids) or small (Small Grids).
Types
When players are talking about grids they are building, you will hear the following words used:
- Subgrid: A grid that is connected to another grid by a mechanical block is called its subgrid. The mechanical blocks are
Rotor, 
Hinge, and 
Piston. Which one is the "sub" and which the "main" grid is relative to the grid whose Terminal you are using. - Multigrid: A grid with one or more subgrids is called multigrid. Subgrids can also have subgrids themselves.
- Main grid: The main grid is the one from where the multigrid is being controlled, the one whose Terminal you are using. If you have cockpits on subgrids, different subgrids can take on the main grid role, depending on which cockpit you are seated in.
- Merged grids: Grids that are attached to each other using Merge Blocks are treated as one grid. Upon release of the Merge Blocks, they can under certain conditions become two separate grids again.
- Supergridding: An old exploit where players edited block definitions in the save file and changed their grid size, resulting in a mix of large and small grid blocks within the same main grid.
Advantages of multigrids
Subgrids allow you to build cool functional things, such as folding solar panels, large sliding or folding hangar doors, vehicle trailers on hitches, rotating VTOL thrusters, foldable mining rigs, walking mechas or robots, flapping wings, cranes, catapults, rotating multi-turrets, fold-out stairs, elevators, escalators, hull and wing shapes with arbitrary block angles, ... and many more functional contraptions or creative decorations!
Subgrids receive power from the main grid.
All mechanical blocks (except the basic Rotor) have conveyor capabilities and you can pipe items through their inventory across subgrids.
By default, subgrids are excepted from a lot of the damage mechanics, because a lot of their movements next to other subgrids would be considered destructive collisions by the physics engine. In the World Settings, you can re-enable Sub-Grid damage if you want to live dangerously.
How to combine large and small grids?
Small grids have the same functionality as large grids but are cheaper and more compact. Alas, blocks of different grid sizes cannot be attached to one another directly. However, both Rotors and Hinges allow the attachment of a small grid rotor head to a large grid stator, which lets you indirectly attach grids of different sizes to one another.
How to build mixed-size grids:
- Either, move a small grid with a rotor or hinge head on it close to a headless large grid stator of the corresponding type, then click the Attach button in the control panel of the large stator.
- Alternatively, construct the large rotor or hinge, grind off its automatically-generated large head, and then click the Add Small Head button in the control panel of the large stator. A small grid may then be built out from the new small head.
The end result will be a semi-permanently attached small subgrid on the large main grid.
Read more under How to connect small and large grid blocks.
Example: A Monitoring Dashboard
You can use Event Controllers and coloured lights and LCDs to build a dashboard that monitors the power/cargo/oxygen/fuel status of your base.
The problem is that on large grids, every tiny light etc. takes up one large grid cube, which makes building a monitoring dashboard unwieldy. Instead, you build a small grid dashboard and attach it through a Rotor.
Similarly, if your large grid ship is using a lot of unwieldy Timers and Event Controllers, you can build (some or all of) them together on a small grid and attach it.
Disadvantages of Multigrids
Knowing what a subgrid is is relevant to an engineer because of its limitations:
Why can't I steer the Subgrid?
The first disadvantage is that you cannot steer wheels, thrusters, or gyroscopes on subgrids from the main cockpit -- unless you use scripts (see below).
You will not be able to use the WASD keys to propel any subgrid wheels, nor fire the subgrid thrusters, nor will the mouse pitch/yaw/roll the subgrid gyroscopes. A wheel attached directly to the main grid through a Wheel Suspension can be steered from the main cockpit, but not if the Wheel Suspension is attached to a subgrid on a rotor/piston/hinge. You can passively drag a wheeled subgrid along, but it will not help you with propulsion, and often its passiveness and extra mass may even hinder steering.
Why is my Multigrid sinking?
The second disadvantage is that Inertial Dampers do not account for the mass of subgrids.
This issue is most strongly noticeable on planetary ships flying in gravity when they use mechanical blocks in landing legs, nacelles, angled wings, vector thrusters, etc. If your ship has subgrids, it will sink[1][2] or tilt in planetary gravity, and you will have to constantly fight that miscalculation manually.
While thrusters on the maingrid don't waste fuel while not actively flying, thrusters on subgrids fight the main grid and do waste fuel, so switch them off after landing or use a script.
Subgrids tilting can be counteracted by a Gyroscope override, but to fight the constant sinking, you’ll need extra thrusters and a subgrid control script (see workarounds below).
Why is my Multigrid Clanging?
The third disadvantage is that subgrids can “clang”: Clanging means blocks get stuck in impossible positions and slowly shake themselves apart due to phantom forces. This happens, for example, when crane arms, elevators, or foldable doors push into other blocks.
Clang also happens when the physical collision shape (hitbox) of a block is not the same as the shape that we see rendered in game. While building, it looks as if e.g. piston-moved blocks just fit next to one another, but to the physics engine, they overlap and push into one another in impossible ways, which later causes destructive phantom forces. Many oversized hitboxes were fixed in Version/1.196, allowing for more dynamic mechanical builds.
Workarounds for subgrid limitations
If wheels or thrusters on a subgrid of your vehicle need to be fully functional, you will have to run a subgrid control script to be able to steer.
You can get helpful scripts from the Steam Community Workshop (currently, PC only), here are some examples:
To prevent subgrid clanging, leave wider gaps between subgrids, for example, by using only thin blocks such as hangar door blocks, armor slopes, or armor half-blocks near subgrids.
To prevent subgrid sinking, lock the subgrids down temporarily with landing gear (or merge blocks) to add their weight to the main grid.
To prevent subgrid tilting, add a gyroscope to the subgrid and set it to a 0 RPM override.
How can I tell what is a subgrid?
You can tell the difference between main and subgrid (relative to the block or cockpit, that you are interacting with) by looking at the colours of the text in the user interface:
- Enter the main cockpit of the grid from which you want to steer.
- Then open the Control Panel of the Terminal.
- When you look at the coloured names in the list of functional blocks, you see:
- white: Functional blocks on the main grid. Steering with WASD and mouse works for these out of the box.
- orange: Functional blocks with one mechanical block separating them from the main grid. They are on the first subgrid, steering them requires scripts.
- yellow to green: Functional blocks attached through additional mechanical blocks. The shade changes with each mechanical block separating it from the main grid. These are secondary, tertiary etc. subgrids, steering them requires scripts.
Note: By "main grid" we mean, on the same grid as the cockpit you are using to view the control panel.