fundamental vs rule of thumb

fundamental

• A main or major principle, rule, law, etc. which serves as the foundation or basis of a system; an essential part 

• The lowest frequency of a periodic waveform. 

• The lowest partial of a complex tone. 

• Related to a foundation. base, or basis; serving as a foundation. 

• essential; extremely important 

rule of thumb

• A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge. 

• Approximated, guesstimated.