1.16/20w06 update: It seems like the 1.15 version of the mod works as well, as long as you update the Fabric API (hint: https://www.curseforge.com/minecraft/mc-mods/fabric-api/files/2875599)
There are versions for Forge 1.12, Rift 1.13, and Fabric 1.14/1.15. Please use the Files tab to select the one you want, the Download button will always give you Forge 1.12 unfortunately.
Sometimes you want to place blocks in a regular pattern, so you need to count blocks, and if you mess up once, the error propagates to all subsequent blocks.
This mod displays a grid-like overlay over the world, with configurable spacing, so the blocks you're interested in get highlighted automatically.
For example, you're building a railroad and want to place a powered rail ever 20th block. Type /grid 20, walk to the first powered rail, type /grid here (or press the c button, can be configured), and the mod will highlight every 20th block from where you're standing.
Or, if you want to place torches in an area to prevent spawns, type /grid 5 - this highlights every 5th block; placing torches on each of them will light everything up enough.
You can switch between block mode (to highlight blocks to place stuff on) or line mode (to display borders; for example /grid chunks followed by grid lines will turn Grid into a chunk border highlighter), have the y-coordinate of the display floating with the player or fixed, and choose different X and Z spacings if you want that.
/grid show and /grid hide: show/hide the grid, bound to the B key.
/grid here: set the origin of the grid to where the player is standing. Bound to C (for center)
/grid blocks: set Block mode: show blue squares above marked blocks
/grid lines: set Line mode: show yellow lines to the N/W of marked blocks
/grid chunks: set X and Y spacing to 16 and the origin to chunk borders. Use together with /grid lines to get a chunk border display that's less intrusive than F3-G
/grid <n> <m>: <n> and <m> are numbers; set the grid spacing to n in X direction and m in Y direction
/grid <n>: set the grid spacing to n in both horizontal directions
/grid fixy: toggle between floating the y coordinate of the grid with the player, or fixing it to where the player stands when you use the command. Bound to the Y key.
/grid spawns: show possible mob spawn locations
/grid circles: shows circle overlays. In line mode, this shows regular circles; in block mode, it tries to find and mark the blocks you should build on to get a circle. Obviously, this can't work well with very small diameters, for example, trying to mark a 3x3 circle will give you a square.
/grid distance <n>: sets the range in blocks, that the grid is rendered around you. If you use very large spacings/circle diameters, you need to adjust this too. Be careful though, high values for distance absolutely kill your framerate when you turn spawn locations on.
/grid biome <name>: draw a marker on all blocks in a certain biome. For example, /grid biome river helps when you want to build a drowned or squid farm. (Only with 1.15).
All keys can be reconfigured using the standard Controls UI.