Interrupts resolution point which happened within the casting time of Instants or Sorceries. So you could cast an Interrupt "while" an Instant or Sorcery was st

Interrupts resolution point which happened within the casting time of Instants or Sorceries. So you could cast an Interrupt "while" an Instant or Sorcery was still casting, but Instants (or Sorceries) had to wait for the current spell to finish casting. This was of course all ditched when MtG switch to a last in, first out stack model with all instants.