These solid brass oil lamp collars are all #1 size, with a threaded opening of 7/8" i.d. The bottom diameter (the part that fits down over the glass neck of the lamp) is 1-3/8" across. If your lamp neck is less than one inch across, select either the acorn or the nutmeg collar instead; if your lamp neck is more than 1-3/8" across, select a #2 size collar instead.

Gem burners also fit #1 collars.