Missing middles

Can you work out the domino pieces which would go in the middle in each case to complete the pattern of these eight sets of three dominoes?

Problem

Missing Middles printable sheet

Here are eight sets of dominoes. Each set should have three dominoes that make a sequence.

Which domino would go in the middle of each set to complete the sequence?

How do you know?

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The first domino has 2 dots and 2 dots. The second domino is blank. The third domino has 6 dots and 6 dots.
 
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The first domino has 1 dot and 2 dots. The second domino is blank. The third domino has 3 dots and 4 dots.
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The first domino has 2 dots and 6 dots. The second domino is blank. The third domino has 4 dots and 4 dots.
 
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The first domino has 1 dot and 2 dots. The second domino is blank. The third domino has 1 dot and 6 dots.
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The first domino has 3 dots and 4 dots. The second domino is blank. The third domino has 1 dot and 6 dots.
 
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The first domino has 2 dots and 6 dots. The second domino is blank. The third domino has 0 dots and 4 dots.
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The first domino has 4 dots and 6 dots. The second domino is blank. The third domino has 2 dots and 4 dots.
 
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The first domino has 3 dots and 6 dots. The second domino is blank. The third domino has 1 dot and 2 dots.