Originally Posted by
penwash
...Some Parker Vacumatic nibs are also nails, but some of them are "springy", which means they will produce a bit of line variation when enough pressure are applied, but it won't be comfortable for prolonged use in this way. As an aside, uncommonly, there are also Parker Vacumatic nibs that are truly semi-flex.
If we use this definition, it would be hard to confuse a springy nib with a semi-flex (or any other flex), which (depending on the quality) will readily and consistently produce line variations.