Why do I have MBR and GPT as partition tables?

Question

I started with Windows and installed Ubuntu as a dual boot. Thus I thought my partition table should be MBR, since this is what Windows uses. When i run gdisk under Ubuntu, this is the output:

GPT fdisk (gdisk) version 1.0.1
Type device filename, or press <Enter> to exit: /dev/sda 
Partition table scan:
 MBR: protective
 BSD: not present
 APM: not present
 GPT: present
Found valid GPT with protective MBR; using GPT.

Why does it say GPT? I boot with Legacy BIOS, not UEFI, thus I thought my Computer must use MBR? What do I miss out?

Answer 1

It says "GPT" because there is a GPT.

While your protective MBR doesn't contain the true partition table, it apparently contains a bootloader code. BIOS doesn't need to know the partitions to boot – it just executes the code at specific (fixed) address inside MBR. Then it's the bootloader job to utilize GPT data.

From Wikipedia:

In operating systems that support GPT-based boot through BIOS services rather than EFI, the first sector is also still used to store the first stage of the bootloader code, but modified to recognize GPT partitions.