Final spacing should be 12" apart in all directions. Since your rows are 18" apart, you can probably get away with 10" apart in the row.

You can keep them at 4" apart when they are little, but when they get a few inches tall and they start to fill in the area, thin them to one plant per 12". The thinnings will be a tender treat. Think of it as an early harvest.

I know it can be hard to pull perfectly healthy growing seedlings, we want them all to get a chance to grow to full size - but be firm! You will have much better results if the plants are not crowded.

Great question! I'll await an answer, also. I was planting my lettuce seeds and the wind picked up and sprayed seeds all over! Now I have tiny little lettuce seedlings EVERYWHERE...and I don't want to hurt them! :-)

Potterhead2's answer is very good. In addition to that, lettuce can be transplanted to better locations when it is 2 or 3 inches tall. That is the one reason I decided to start it in flats. I get nice uniform rows.

Jim

Well, romain doesn't really form "heads" It's a cut and come again type lettuce. I pick 2-3 leaves of each kind of lettuce for a salad for the 2 of us.
I just sprinkle my mesclun mix around the box I'm growing them in and thin them as they sprout if they are too close together.
I also sprinkle about every 3 weeks or so to keep it coming. You really don't need a lot to keep a couple of people in salad!
I'm planting my next "sprinkling" at the same time I'm planting my green beans, so once they sprout, they'll be shaded by the beans.
Have fun! Nancy

Good to know. I just know what I buy in the store which is heads so this is great to know. THANKS!

Romaine will certainly form its characteristic heads if let to grow, but some people like to cut lettuce as it comes along, when the leaves are young.

I don't even worry about how close mine is. I sprinkle the seed in there thick and it grows just fine. In harvesting some get plucked and I pluck a few that are way too crowded but lettuce doesn't need a lot of room and I think it is a waste of space to do the rows with 12 or 18 inches between. If it's a wide open space it's an invite for a weed, I'd rather have lettuce.