Ternary Form (ABA form) Theory Things   William Wieland
Sectional or Closed Ternary (A ends on tonic.)
Twinkle, Twinkle Little Star 
Schumann "Humming Song" from Album for the Young op. 68 no. 3
Both A B A and A B A' are common. Da capo arias are in ternary form.

Sectional or closed ternary means that the A section begins and ends in tonic. In a continuous or open ternary form, the first A section would not end on tonic.

B often ends on V. B contrasts with A melodically, harmonically and/or texturally. B may develop motives or themes from A.

Repeat signs may occur, frequently   ||: A :||: B   A :||

Though A A' A'' and A B C each include three sections, A A' A'' features no contrasting middle section and A B C does not recapitulate the opening material.

Compound ternary is a three-part form in which each section is itself a binary or ternary form. Common examples include Minuet and Trio or Scherzo and Trio movements.