Apple's (NASDAQ:AAPL) annual iPhone event is almost here. Well, technically, we can't be 100% sure that Apple's Sept. 9 event is intended to unveil the iPhone 6. But considering the timing and the numerous leaks, it's fair to predict with 99% certainty that new iPhone models will make their debut.

Here is a summary of the most important rumors about the iPhone 6, along with a few last-minute leaks.

Apple media invitation. Image source: MacRumors.

Two models
Like last year, Apple is again expected to launch two versions of its next-generation iPhone. In 2013, the two models were the iPhone 5c and the 5s. This year, the active Apple rumor mill is predicting both a 4.7-inch and 5.5-inch iPhone. These two sizes are both larger than the 4-inch display on the iPhone 5, 5c, and 5s.

It's not clear yet what these iPhones will be called. Some have speculated that the larger iPhone will be called the iPhone Air, bringing the "Air" designation to the iPhone line for the first time. This would also drop the emphasis on numerical models. If Apple takes this route, it's also possible the iPhone 6 will be simply dubbed "iPhone." But it wouldn't be surprising, even if the larger device is dubbed iPhone Air, if Apple keeps the numerical models and calls its 4.7-inch smartphone the iPhone 6.

Alleged leaked 4.7-inch iPhone 6 rear casing. The part was first shared by Feld & Volk.

The two phones are expected to adopt thinner form factors that resemble the iPad Air and the iPad mini with Retina display. The current thickness of the iPhone 5s is 7.6 millimeters. The 4.7-inch and 5.5-inch iPhones are respectively expected to have a thicknesses of 6.9 mm and 7.1 mm. For further comparison, the Samsung Galaxy S5 is 8.1 mm thick. 

The displays for the iPhone 6 models are expected to be made from a sapphire crystal material. But several reports have suggested Apple might not have been able to ramp up supply rapidly enough to use the new material in its larger phone. On the other hand, there are reports every year about supply bottlenecks that threaten to limit production and cause delays. So don't be surprised if the sapphire crystal shows up in both models.

The guts will be revamped as well. Both phones will likely sport Apple's next-generation ARM-based system on a chip, likely to be referred to as A8.

iPhone 6 concept. Design by Tomas Moyano and Nicolas Aichino.

New rumors
Rumors continue to percolate about Apple using near field communication, or NFC, technology for a mobile payments service in the iPhone 6. The New York Times reported on Thursday that Apple has already made arrangements with major credit card companies.

The wearable device and the smartphones will include hardware and software that support a technology called near-field communication, or NFC, which allows devices to exchange information wirelessly over very short distances. It could make paying for things with a phone less of a hassle.

Apple has teamed up with American Express (NYSE:AXP), MasterCard (NYSE:MA), and Visa (NYSE:V) to support the payment system, said several people involved in the partnerships. With the deal, these people said, iPhone owners will be able to use their devices as a sort of digital wallet, improving their ability to pay for items at select partner merchants without handing over cash or a credit card.

One of the most recent major rumors involves the iPhone 6 camera. New photos leaked late last week allegedly show the rear camera for the 5.5-inch version. The photos come from Feld & Volk, hands down the most prominent source for iPhone leaks this year.

Alleged rear camera for 5.5-inch iPhone 6 (left) and camera for iPhone 5s (right). Photo by Feld & Volk.

In line with other leaks and reports, the camera for the 5.5-inch version of the iPhone will protrude from the rear shell, according to Feld & Volk. Furthermore, the report asserted that the camera for the larger iPhone will include optical image stabilization.

Launch timing
Since the launch of the iPhone 4s, Apple has launched the new iPhone models on the following Friday after the unveiling, or 10 days later. If the pattern continues, consumers can get their hands on the new iPhone models on Friday Sept. 19.

As the iPhone provides more than half of Apple'srevenue, and is the company's most profitable business, investors hope the latest models are a hit with consumers. While many minor details about the new phones remain unknown, the fact that rumors still point to iPhone models with displays larger than 4 inches is reassuring news for shareholders given that the market for larger smartphones is arguably already proven to be hot.

Daniel Sparks owns shares of Apple. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.