A frontline that holds
Your carry only deals damage while it is alive. Enough bodies in front, and real defensive items on them, buy it the time.
Checked for patch 18.4. The numbers come from the live Set 18 data and follow every patch.
The idea
Your carry only deals damage while it is alive. The frontline's job is to buy it that time: take the first wave of damage, hold the enemy's melee units in place, and eat the abilities that would otherwise land on your back line.
A frontline fails in two ways. Too few bodies, and the enemy walks through and reaches your carry while it is still warming up. Bodies with nothing on them, and they melt in the opening seconds no matter how many you field. Strong boards solve both: top four boards at level 8 usually field 5 frontline units, and 99.7% of them give at least one of those units a defensive item.
Every level gives you another slot, and a good share of those slots belong in front. It is tempting to fill each new slot with another damage dealer, but a second carry with nobody in front of it dies just as fast as the first.
Tanks do not need a perfect build. Armor against the attack damage your lobby plays, magic resist against the ability power, and health on top will do more than any damage item on a unit whose job is to stand there and take hits.
See it live
A current comp's final board, each unit lit by the line the game review puts it on. Defense items are marked, and each unit's card tags its items by class.
Lunar Aphelios Nidalee carries with Aphelios, a 4-cost. It is the most played comp with a 4-cost carry right now. This is its final board.
Front line, 5 units: Sentinel, Amumu, Nidalee, Vi and Diana. Tanks, fighters and melee units.
Back line, 3 units: Aphelios, Kog'Maw and Varus. Ranged units behind them.
Defense items are ringed on the board. Point at or tap a unit for each item's class.
Top four boards at level 8 usually play 5 frontline units, and at level 9, 5. The review only judges a frontline's items once a board has 6 finished items.
The rule of thumb
Field enough frontline for your level, and give at least one of them real defensive items.
Check yourself
How many frontline units does this board play?
Pick an answer.