writes letters on dotted thirds vs writes letters in dotted thirds

Some examples from the web:

• This way if things go south with Holly, I can still get him to write letters on my behalf.
• I can write letters home on it.
• Some of them were kind enough to write letters.
• And that person you write letters?
• Well, your hand is programmed over years to write letters a certain way.
• Unfortunately, I have to write letters like that more often than I would like.
• She uses my body to write letters.
• To remind you to write letters occasionally.
• These women like to write letters to horrible men.
• Grandpa, I really detest people who writes letter and runs away.
• Typically, offenders write letters to be heard.
• What a spot to write letters!
• Readers write letters in response to the ads and mail them in.
• He probably writes letters to The Times.
• So you write letters to the people that you love.
• His parents could at least write letters back and forth.
• To write letters home for the men who have both arms broken.
• I recall Don starting to write letters to them referring to them as Nickel and Dime Records.
• Write letters to Italy and maybe...
• People write letters, kids stand in front of tanks, college students vote.

Some examples from the web:

• Readers write letters in response to the ads and mail them in.
• From what Yule Mae says, there's a lot of time to write letters in jail.
• I can write letters home on it.
• Some of them were kind enough to write letters.
• And that person you write letters?
• Well, your hand is programmed over years to write letters a certain way.
• Unfortunately, I have to write letters like that more often than I would like.
• She uses my body to write letters.
• To remind you to write letters occasionally.
• These women like to write letters to horrible men.
• Grandpa, I really detest people who writes letter and runs away.
• Typically, offenders write letters to be heard.
• What a spot to write letters!
• He probably writes letters to The Times.
• So you write letters to the people that you love.
• His parents could at least write letters back and forth.
• To write letters home for the men who have both arms broken.
• I recall Don starting to write letters to them referring to them as Nickel and Dime Records.
• Write letters to Italy and maybe...
• People write letters, kids stand in front of tanks, college students vote.