When something is rudimentary, it is in its basic form or not developed
"Consisting in first principles, of a primitive kind, very imperfectly developed or represented only by a vestige"
New York Times- As Earth Warms, Old Mayhem and Secrets Emerge From the Ice
“A freshly unfrozen alpine pass at Lendbreen — in use from about 600 to 1,700 years ago — yielded evidence of the tradespeople who traversed it: horseshoes, horse dung, a rudimentary ski and even a box filled with beeswax.”
BBC News- Africa's iconic architecture in 12 buildings
"I always found it interesting that people would use what was around them to turn a rudimentary structure into a piece of art."
U of A Division of Agriculture- "Once Thought Rudimentary, Complexities of Mosquito Hearing May be Useful for Entomologists"
-ary: relating to
Rudiment
Rudimental
Rudimentary came from the Latin word rudimentum, which means beginning (from the word rudis, meaning rude or raw). It also came from the French word rudiment in the 1540s.
The word rudimentary has changed a little bit from its Latin root word. It started out meaning beginning or raw, but has since changed to describe something basic or elementary.