I couldn't find this answer anywhere on the internet:

why are some microphones called "line + gradient"?

thanks

It refers to the polar pattern of a particular design of shotgun mike, which is described here:
Audio-Technica - Microphones, headphones, wireless microphone systems, noise-cancelling headphones & more : What's The Pattern? (http://www.audio-technica.com/cms/site/aa901ccabf1dfc6b/index.html/) /hth/ Battle Vaughan / retired videographer

Thanks for the response!

So the "line" part means shotgun mic, and the "gradient" means directional

Here's what the link said:

When miking must be done from even greater distances, line or "shotgun" microphones are often the best choice. Line microphones are excellent for use in video and film, in order to pick up sound when the microphone must be located outside the frame, that is, out of the viewing angle of the camera.

The line microphone uses an interference tube in front of the element to ensure much greater cancellation of sound arriving from the sides. Audio-Technica line microphones combine a directional ("gradient") element with the interference tube to increase cancellation at the rear as well.

Thanks for the response!

So the "line" part means shotgun mic, and the "gradient" means directional

Here's what the link said:

When miking must be done from even greater distances, line or "shotgun" microphones are often the best choice. Line microphones are excellent for use in video and film, in order to pick up sound when the microphone must be located outside the frame, that is, out of the viewing angle of the camera.

The line microphone uses an interference tube in front of the element to ensure much greater cancellation of sound arriving from the sides. Audio-Technica line microphones combine a directional ("gradient") element with the interference tube to increase cancellation at the rear as well.

More accurately, "line" refers to the use of an interference tube (the long 'gun-barrel' with the holes in it) that gives the shotgun mic its name. "Gradient" is short for pressure-gradient. Mic transducers, the gizmo that converts the mechanical motion of air molecules into the electrical signal, fall into two categories, pressure and pressure-gradient. The diaphram of a pressure transducer covers one end of a sealed box, like a kettle-drum, and moves in response to the rise and fall in air pressure that is the passing sound wave. Omni mics are pressure transducers. The diaphram of pressure-gradient transducers moves in response to the difference in pressure between the front and back of the diaphram. Cardioid and hypercardioid mics use pressure-gradient transducers. Line + gradient mics put an interference tube in front of a pressure-gradient transducer.