Twinkle bulb

These are easily recognizable by their red tips on an otherwise unpainted clear white bulb, and are available in the different voltages required for 10, 20, 35/70, and 50/100/150/200-light sets.

Some older mini-light sets interlaced two circuits, putting all of the odd-numbered bulbs on one and the even-numbered ones on the other, allowing some lights to remain on while others between them blinked off.

Some sets of 100 did this, others had the more common sequential (non-interlaced) setup but were advertised as "5-way flashing", having five circuits of 20 bulbs.

When first turned on, the strip does not make contact, causing power to flow through the filament until it warms up within a few seconds.

LEDs also have twinkling versions, though they contain an oscillator that alternates on and off at a very regular intervals rather than the irregular blinking of the thermal bimetallic strips.

Others have a more advanced control within the LED that can vary the brightness like a candle flame, or fade between two or three colors, but these are not twinkle bulbs strictly speaking, and are often used individually instead of in strings.

Synchronized flashing or color changes with a control box are not due to twinkle bulbs, but LED ones may have two colors wired in inverse parallel within each bulb to respond to changes in electrical polarity (and voltage for brightness) from the control box.