As you can see in the added picture to the thread:

**Picture 1:**
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I have attempted to say I am going to write on one line and then drop the next line; then write again on the third line and so on. Please let me know which option sounds incorrect and why?

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> 1.a. I am writing **every other line.** (I don't know if I have to use the preposition "on" here or not.)   
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> 1.b. I am writing **every second line.** (I don't know if I have to use the preposition "on" here or not.)   
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> 1.c. I am writing **on alternate lines.**
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**Note:** to me all the three choices above work naturally and mean the same thing.

**Picture 2:**
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Now, let's say you are going to write on the first line, then then drop the **next two lines**; then write again and so on. I wonder which option doesn't work and why:

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> 2.a. I am writing **every three lines.** (I don't know if I have to use the preposition "on" here or not.)   
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> 2.b. I am writing **every third line.** (I don't know if I have to use the preposition "on" here or not.)   
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**Note:** to me all the two choices above work naturally and mean the same thing.

Please have a look on my picture and let me know if I have been right or wrong in my options.

Also please kindly correct my mistakes if I have been incorrect.

![enter image description here](https://i.stack.imgur.com/oCDqQ.jpg)