US Patent 7575385 Double-head writing instrument

A double-head writing instrument has a barrel having a partition formed inside thereof, first and second ink-tanks provided inside the barrel so as not to communicate each other, first and second ink-retaining members connected with first and second nibs provided on an opening of the first and second ink-tanks, respectively, first and second ink reserved inside first and second ink-reservoirs, respectively. The partition is extended in an axial direction so as that the first and second ink-reservoirs overlap each other in a radial direction, and thickness C of the partition and thickness D of a side wall of the barrel connected with the partition satisfy a relation of 0.2≦C/D≦2.5.