Hello,

Part numbers that you find at the back of LEGO instructions are actually a number that is unique for a part design + colour (or decoration). On the contrary, there is also a number that is embossed in the real pieces and that corresponds to the design only. Therefore it is the same for a 2X2 Black brick and a 2X2 Red brick. This is the number that mecabricks - and the other software - is using.

This number is accessible in the inventory of websites like brickset, bricklink or rebrickable for example.

This is an overview of how the system is working but you have heaps of exceptions especially with decorated parts, parts we don't know the design ID because it is not embossed on real parts, parts that are actually an assembly of multiple parts, etc.

I'm following as much as I can the LEGO part numbering whenever it is possible.

okay Thank for the info so what would be the best work flow for finding a part for a model..I wanted to build metal beard and was using instructions to a duel set but the part nums werent making.

should I use the part nums in the lego instructions and then find them on the sites you mentioned and then take that part num and look to see if you have them made?

I kinda know every single part from the library as well as where they are so I'm a bit biased 😉
You'll probably learn where the parts are after playing a bit with mecabricks and you won't need to search for too long. What I like best is to have the real instruction book (or pdf but it is not as practical) and brickset.com opened at the page of the set - select tab Parts to access the inventory. Like that it can help you if you are unsure about which colour to use or which part design it is (number with 4 or 5 digits).

Great I'll give that go. Thxs!