On Tuesday, Apple (NASDAQ:AAPL) will report third-quarter results. The quarterly update will be an important one for investors, providing a helpful glimpse at some key areas investors are uncertain about -- particularly Apple Watch and iPhone sales.

Apple Watch
With the first Apple Watch deliveries beginning in April this year, the company has yet to report any meaningful information on how the Apple Watch is selling. Apple's third fiscal quarter, which began in April, marks the first quarter including Apple Watch sales. So, investors will likely finally get an Apple Watch unit sales figure to mull over.

Apple Watch Tmf

It's difficult to overstate the importance of the Apple Watch's performance in the market during its first quarter of availability. After all, the company's launch of Apple Watch marks the first new product category introduced under CEO Tim Cook's reign. The last time Apple entered a new product category was when it launched the iPad in 2010, under the leadership of former CEO Steve Jobs.

iPhone
Can the wildly successful iPhone 6 launch keep up its impressive pace? Investors will be looking to see whether historical patterns of seasonality for iPhone unit sales will play out similarly for the company's latest iPhone. The iPhone 6 and iPhone 6 Plus launch has been the company's most successful iPhone launch yet -- by a long shot. 

Historically, iPhone sales typically begin to pull back considerably in Apple's third quarter. This has to do with both the launch timing of the iPhone and seasonality. Given that Apple's first quarter typically marks both the first full quarter of sales for its new iPhone models and falls during the holiday season, the tech giant's first-quarter sales are usually monstrous compared to the quarters before it. And iPhone sales are typically markedly lower in the three quarters that follow it.

Iphone

iPhone 6. Image source: Apple.

In the third quarter of 2014, iPhone sales declined 19% sequentially. If Apple can maintain this same rate of decline between Q2 and Q3 this year, the company's third-quarter iPhone unit sales would be around 49.6 million. This would be up about 41% from the year-ago quarter.

As it turns out, the consensus estimate for iPhone unit sales in Q3 is 49.4 million, according to a survey of Apple analysts by Fortune.

For reference, Apple's iPhone unit sales declined 18% sequentially in Q2, but were up 40% from the year-ago quarter.

Apple's third-quarter financial results will be released shortly after market close (usually about 15 minutes after close) on Tuesday, July 21. The report will be available to view here. Apple will host a conference call at 2:00 PM PST on the same day, which anyone can tune into by visiting this URL.

Stay tuned at The Motley Fool for more pre-earnings Apple coverage as well as analysis of the company's results after they are posted.

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.