The English (or Shakespearean) Sonnet is a poem form consisting of 14 lines, each with 10 stressed and unstressed syllables known as iambic pentameter, with a set rhyme scheme of: 
                      a b a b c d c d e f e f g g 
    The rhymes may be ear-rhymes or eye-rhymes: an ear-rhyme is one that rhymes in sound, e.g. “increase” and “decrease”; an eye-rhyme is one that rhymes by sight, e.g. “compare” and “are”. 
    This rhyme sequence sets the usual structure of the sonnet as 3 quatrains (sets of 4 lines) concluding with 1 couplet (a pair of lines). It is usual for there to be a pause for thought ("the turn") in the sonnet’s message at the end of each quatrain, especially the 2nd., in order to add tension, with the sonnet resolving to its objective in the final couplet, just as a song normally resolves to its root chord at its close. To convey the sense of resolution and completeness at the end of the sonnet there are often key-words, or tie-words, present in the closing couplet that are also present in the earlier quatrains. This structuring provides a framework on which to build the words, phrases, themes, rhymes, syncopation, punctuation and rhythm of the sonnet making it, at its best, a self-contained work of art. 
        Having established this structure though, the author can then go on to breach the framework to add tension and meaning: 

• A quatrain will not necessarily comprise a full sentence - instead a quatrain may contain more than one sentence or a sentence may straddle more than one quatrain, sometimes extending across the whole sonnet giving a breathlessness to the sonnet. 
• Sentences may end mid-line adding tension and dysfunction to complement the message of the sonnet. 
• Key-words may be deliberately absent from certain quatrains where they don’t belong with the sentiments of that particular quatrain, emphasising the meaning the author wishes to convey by literally being absent. 
• In this way, the sonnet can convey meaning and mood via compliance with the standard sonnet structure, or by deliberately venturing outside of that framework, depending on the effect required. 

         The sonnet was introduced to England by Sir Thomas Wyatt (1503-42) following his various European diplomatic positions, including Italy, in the court of Henry VIII. The form was then developed by Henry Surrey (1517-47) and became very popular with several Elizabethan sonneteers, particularly during the 1590’s, among them Shakespeare. 
                            From The Place 2 Be