The hexode and heptode

In some older radio and TV receivers, tubes with 4 or 5 grids were used sometimes. These tubes had 6 and 7 elements, respectively, and were called as hexode and  heptode. The usual function of this type of tubes was signal mixing. The schematic symbol for a hexode is shown in the figure given below; the symbol for a heptode is illustrated at D.

You will never hear about these devices in the modern electronics, as solid-state components are used for signal mixing nowadays. You may elicit a raised eyebrow if you talk about a pentagrid converter or a heptode mixer.