सम

सम
स Equal, gather, furrow
म Inside, fish
Hence
Compound, together
Equal inside
Internally Equal
Inside furrow
gather(ed) inside

Because ञ न म ण ङ change their form depending on subsequent वर्ण, सञ preceeding चु (च छ ज झ), can be सन सम सण or सङ. Same for सङ preceeding कु (क ख ग घ), सम preceeding पु (प फ ब भ), सन preceeding तु (त थ द ध), सण for टु (ट ठ ड ढ).
In this context, consider the following -

सङ = स ङ = gather outside, parallel outside, straight/align outside, furrow outside
सञ = स ञ = (gather, furrow, parallel, straight) x (surface, hyde, cloth, net)
सन =स न = (gather, furrow, parallel, straight) x (noose, pull,, below)
सम = स म् = as described above
सण = स ण = (gather, furrow, parallel, straight) x (push, shovel)