In Interlace mode a secondary surveillance radar interrogates the aim alternating in two or more different modes of the older one Mark X standard. The simplest interlace mode is the Mode A/C, i.e. the SIF-system interrogates mode Alpha (identification) and in the following pulse period the interrogation mode Charly (barometric height).
In military IFF a so called three- interlace mode is even in use. In reality the military IFF should to interrogate up to five different modes quasi-simultaneous. This brings problems with the available time, because the SSR interrogator don't use every pulse period of the primary radar to avoid interferences with other SSR (so called: Fruit). More than three different modes need about 8 to 12 pulse periods of the primary radar. This has the disadvantage, that the dwell time of the radar could be too short for more mode requests. Therefore the military interrogator transmits after mode Alpha and mode Charly in the third SSR-pulse period the first military mode 1. In the next scan (i.e. in the next antenna revolution) it transmits in the third SSR-pulse period the second military mode 2 instead the mode 1 and so on.
In the more modern mode-S the secondary radar transmits single selective interrogations and overcomes the problem with the different mode requests.