Design ID vs. set number
Set numbers (like 10305) identify boxed products. Part numbers (often four or five digits, sometimes with a letter suffix) identify the mold — the physical shape of the element. One mold can appear in hundreds of sets in many colors.
Confusing the two is a common seller mistake. Always identify the mold first, then note color separately when pricing or listing.
Where to look on the piece
Turn the element over or look inside tubes and studs. LEGO typically molds a tiny number on the underside — it may be hard to read on black or dark gray parts without angled light.
Technic pins, clips, and small plates sometimes hide numbers along an edge rather than under a stud. A phone macro shot or a loupe helps for elements smaller than a 1×1 plate.
Suffixes and variants
Letters after a number often indicate a mold revision or a variant that shares most of the same geometry. Treat suffix differences seriously when matching printed or dual-mold parts — the catalog treats them as separate items.
If two numbers look close but the shape differs (for example, with or without a groove), trust the geometry over memory and search both IDs.
Using numbers in search
Paste the molded number into BrickID's search field. If nothing appears, try without suffix letters or search by shape keywords ("2x2 jumper", "1x2 grille tile").
Once you land on a part page, open the parent-set list to see every official appearance. That list is what you need for pricing rare colors or retired prints.