head shown in fig6 does not require any extra mass for counterbalance the housing 112 for this second embodiment has two recessed cavities 160 a and 160 b see fig8 each for receiving a pivot post 116 ignoring the orientation of the pivot posts the head 110 is symmetrical about a vertical plane passing through the vertical central axis of rotation of the two pivot posts and symmetrical about a vertical plane perpendicular to the first plane which also passing through the midpoint between the two posts fig7 shows a bottom perspective for the second embodiment trimmer head 110 the hexagon cavity 136 is the same for both trimmer heads 10 and 110 however they could be designed differently for example they could be designed to work with a larger nut or bolt size there are also other methods for attaching a trimmer head to a string trimmer machine which are commercially practiced these alternative methods could likewise be used additionally the housing could also have a perimeter shape other than round the perimeter could be oval square or rectangular with rounded corners three legged like the ugly u2122 head or any of a variety of head shapes it is also possible to have two or more pivot posts that are not equally spaced so long as mass is added to balance the head to prevent vibration during rotation the next discussion will reference fig5 10 11.