From the 1900's through the 1940's Conn would describe the bore sizes of their cornets and trumpets as "small", "medium" or "large". I have found that these descriptions are very consistent, but differ between trumpets and cornets. Here is a table to translate these descriptions into acutal bore sizes. Conn's numbering system continues beyond 2½ up to (at least) 5, but since this table focuses on cornets and trumpets the higher numbers are ommited here.
In later years the bore sizes and the numbering system remained the same, but the description were discontinued, possibly because design changes caused instruments with the same bore to feel very different.
| 1900's - 1940's Conn Bore Size Descriptions | ||||
| Conn Bore Number | Bore Size Inches | Bore Size Millimeters | Conn's Cornet Description | Conn's Trumpet Description |
|---|---|---|---|---|
| ½ | 0.422" | 10.72 | - | - |
| 1 | 0.438" | 11.11 | - | Small |
| 1½ | 0.458" | 11.64 | Small | Medium |
| 2 | 0.468" | 11.91 | Medium | Large |
| 2½ | 0.484" | 12.30 | Large | - |