Pen Sac - Size 23 Necked. 23/64 inch diameter necked latex pen sac mainly for Parker Duofold fountain pens. 

Pen Sac - Size 21 Necked. 21/64 inch diameter necked latex pen sac mainly for Parker Duofold fountain pens. 

Pen Sac - Size 19 Straight. 19/64 inch diameter latex pen sac, a popular larger size used in Sheaffer, Waterman, Parker and many other fountain pens.

Pen Sac - Size 17 1/2 Necked. 17/64 inch diameter necked latex pen sac mainly for Parker, Eversharp, Sheaffer fountain pens. 

Pen Sac - Size 17 Straight. 17/64 inch diameter latex pen sac (one of the most popular sizes) for many fountain pens, including Sheaffer, Parker, Waterman, Eversharp and others.