In Hebrew poetry the usual form of parallelism is a pair of lines that are roughly the same in meaning (synonymous), or that present a contrast (antithetical). In Psalm 6 we see nine verses in the standard form, and all of these except verse 3 are synonymous. But one verse (v.6) adds a third line to form a triple parallelism.
I am weary with my groaning; All night I make my bed swim; I drench my couch with my tears.
Psalm 100 is unusual in that it consists of four triple parallelisms. Verses 1 and 2 make up the first, and each verse thereafter another. All of the parallelisms are synonymous. Verses 1-2 are a call to praise, followed by reasons for it in verse 3. Verse 4 is another call to praise, followed by reasons for it in verse 5. Thus, the larger structure of the psalm is in an ABAB form. This is a double parallelism at a higher level: verses 4-5 are parallel with verses 1-3.
A Psalm of Thanksgiving
1. Make a joyful noise to Yahweh, all the earth.
2. Serve Yahweh with gladness.
Enter to his presence with shouting.
3. Know that Yahweh, he is God.
He has made us and not we ourselves.
We are his people and the flock from his pasture.
4. Enter his gates with thanksgiving,
His courts with praise.
Be thankful to him. Bless his name.
5. For good is Yahweh.
To everlasting is his lovingkindness,
And to generation after generation is his faithfulness.
The symmetry of the psalm suggests the law-abiding orderliness and carefulness of our drawing near to God, while the psalm itself calls us to exuberant praise. Overflowing joy and careful adherence to the commandments go hand in hand.