For example, if you follow the recognised process of working from the known to the unknown and you have a John Smith as your grandfather, you obtain the mattiage certificate for John Smith and your grandmother Mary Jones. On that certificate is unique inormation like their age, location at marriage, fathers names and names of whitnesses and their religion/parish.
From this you establish details that will eliminate other John Smiths and Mary Joneses, you enter the known information into your tree and then create an action plan that narrows down the possibilities, so, If John Smiths father is given as John Smith and he married in 1950 aged 21, you are looking for a birth of a John Smith in 1929 with a father John Smith BUT that would be a tough search unless you had a location (registration district) or the mothers maiden name, and so the distaff (maternal) side becomes a more inportant collaborating fact than normal. One often overlooked rersearch option is Monumental Inscriptions, these provide names dates and relationships all in one location.