parallel with/to

“The flags are not parallel with/to the ground.”
Which is the correct preposition for this sentence?

It is ‘parallel to’.

Lines which follow the same course run parallel with or parallel to each other. Both are accepted as correct unless you adhere strictly to very traditional, archaic rules.

You might also say, “The flags are not in parallel with the ground,” as an idiomatic expression. In that case, ‘in parallel to’ would be wrong.

I must say though, that unless the flags in question were fastened by both top corners and pulled very tightly so that the tops were rigid, I would not expect them to be parallel with the ground!